Shop Template/Trouble with Add to Cart option

I have received many complaints from customers, regarding the lack of an “Add to Cart” button when they choose their preferred size. I have a feeling this is an issue mostly for mobile device users, however I do not want to limit my clientele to a PC/Home unit only when ordering from me. I once did an update to the current theme being used, this seemed to be a resolve, but now I have customers reporting the issue again.

If any of this seems confusing, or needs more detail, I will try to go into more depth.

I am using the “Boutique” theme

Thank you greatly in advance.