My store speed needed improvement so I hired an expert via shopify a few months back. They improved the speed and all was fine. I paid for monthly store speed maintenance since then. I changed my store a lot since the initial optimization so of course the speed went down.
However, since having this maintenance done my store speed fluctuates so much even if I do nothing for many weeks. It goes up and down by 6-10 points sometimes. I keep being told that store speed changes but constant up and down? Could it be caused by an app? Today my store speed is 33
My theme is Fashionopolism. Link is https://bajalondon.co.uk/
Fluctuation is common, the Shopify speed score imports from Google Lighthouse (same engine as Google PageSpeed). You'll often get different scores from run to run without changing anything.
Also, if you have any 3rd party apps installed that are store facing, the speed of the resources those apps load may depend on both your server + their server and the current bandwidth, so it's not always going to be the same.
If you don't change anything on your store, you don't need monthly maintenance. There's nothing to maintain, code does not change. The only maintenance you potentially need is if you delete an app that added code to your theme, you'll have to remove that code after you delete the app.