Why are some customers experiencing checkout issues on my website?

[Maisahmakes]

Hi. Recently my customers are having issues with checkout. My website picks and chooses who's order wont go past checkout and whos will. I had a drop today, some customers experienced issues, others didn't.

All shipping profiles are correct

all products are assigned to shipping profiles

Its completely random - one order will come through fine from UK, another wont. and the same for international too. 
The errors are mixed too - some are "doesnt ship to your country" - but I do

other error is - "some items are not available" - again they are. 

Anyone else experiancing issues? I've had this since the Markets was introduced, and I dont have an option to deactivate it like everyone else?!

[PluginHive]

@Maisahmakes Are you using any fulfilment or shipping apps?

The first thing I'd suggest is to ensure that you've set up your shipping to include your desired shipping zones. Customers can order products from your store only if the country or region where they want the order shipped to is included in one of your shipping zones. 

Go to Settings—Shipping and Delivery -Manage Rates (set it up correctly)

If this is already set up correctly, the error may occur as a result of missing gaps between the different shipping rates or having overlapping weights.

If it is still not resolved, kindly contact Shopify live chat support: https://help.shopify.com/en/questions#/contact/chat