I am using the Shopify theme Craft with no issues, then the other day I was informed that no matter what size my customers choose, it adds to the cart as the smallest size, but only on mobile not on desktop. When I check the mobile view on my desktop it’s working fine, it’s only when I’m actually using my phone, so I can’t check the code and have no idea what to do, please help! Here’s one of the products it’s happening with: The Daisy Trail Bow Mini Dress – The Octopus Garden
The issue is occurring because your theme is not handling the size change properly on mobile devices.
In simple terms:
When a customer selects a size, the theme tries to update the product details on the page.
On mobile, this update process breaks because some required elements are missing or don’t load correctly.
That error stops the variant from updating, so Shopify can’t register the selected size.
When that happens, Shopify automatically adds the default (smallest) size to the cart.
So the real reason is:
A mobile-only theme error is stopping the variant from updating, so the store falls back to the first available size.
I think I can see what’s happening and its to do with the speed of everything that loading on your product page.
If you click add to cart before everything has completed loading, the code (javascript) that handles variant selections isn’t running yet, and so it defaults to the small variant size.
I took a quick look and there’s a couple of files that your page is trying to load from “instagrid.instasell.co.in”, this is taking 30+ seconds and then failing, and this is slowing things down generally on your page.
So if you can try and remove that from your theme, it might help alleviate the problem.
Beyond that, I think your theme would benefit from a developer spending some time optimising it further, especially with how variants are being handled.
Thank you for your advice! Why would this only be an issue on mobiles? I’m so bad at this stuff and I’m so confused this has only just happened after using this theme for years!
@jpaylor is spot on – you have an app from Instasell – looks like this particular app is no longer listed, maybe they no longer support it?
The app code blocks your page from rendering fully and working properly for a long amount of time.
Say, when I look at your page on desktop, I see nothing below the “Share” button – no “Related products”, no Footer for about 25 seconds.
This can be even longer on mobile.
Enough time for people to click the size selector and then add to cart, but the page is not working properly during this time.
You need to find and either remove or disable the app – at least find the apps block in footer and turn it off and see it this will make your site quicker to respond to size selection…
Should be similar to this screenshot (I have “Forms” block, you should have Instagrid one in your Footer) – it would be safe to delete since it does not seem to work.
And no, there is only one variant selector for both desktop and mobile on your page.
Installing yet another app is not the proper way to fix this.
To clarify, it isn’t only an issue on mobiles - its just that mobiles can potentially load the page slower so the problem is exaggerated / more likely to show there, and it could be that you’ve not viewed those pages recently on your mobile so it hasn’t cached some of the files it is loading (which helps to speed up the page).
I can actually see the problem on desktop too, and I am guessing other visitors to your site will experience the same.
As @tim_tairli says, if you can find a way to remove that app, it will likely improve this if not resolve it altogether for anyone visiting on desktop or mobile.
These are the files causing a problem on your theme:
https://instagrid.instasell.co.in/_/pd/index.js
https://instagrid.instasell.co.in/_/pd/index.css
If the app is no longer installed but these files are still being served, it may be that the app modified your theme’s code directly (older Shopify apps used to do this), and it has left behind some code / references to this.
If so, you’d need to edit the theme code - looking for references to those file names and removing them.
The code to remove looks like this:
You may find this in Online Store > Edit code
Before you do this, you can duplicate the theme from the same menu to keep a backup.
When you choose to edit code, there is a search so this should help you to locate what needs to be removed:
If you’re not comfortable doing this, it would be a good idea to ask a web developer to help you. Or you could try reaching out to Instasell for help seeing as their app caused this problem, their details are here: InstaVid - Shoppable Videos for Ecommerce Brands
Thank you so much both of you for your help so far! Nothing comes up for ‘instagrid’ when I search in my theme code so I don’t know where to find this, and I have removed the app from my footer and it’s still running super slowly. So frustrating!
Sidekick has looked through my whole code and can’t find anything related to Instasell or Instagrid, and running my site through PageSpeed Insights shows it could be Google Tags Manager, which also isn’t installed?! I’m so confused!
Oh it’s still having the same issue running super slow on mobile for me! 
Looks like you’ve managed to remove the app?
The page is more responsive now, I’d say the original problem should be fixed…
I’d recommend making a pause now – do not try to fix everything at once, especially, since there is not much to fix.
Its super quick now for me, so it could just be caching on your phone loading the old version still. It’ll catch up eventually but you could try closing any open tabs on your phone on the site and visit it again in a new fresh tab.
Thank you so much for your help! I’ve just checked and it’s all working on Safari on mobile now but still not on Ecosia?! I will try clearing the cache, thank you so much again 