Mobile issues with quantities

I'm having issues with quantities in the shopping cart on mobile devices. On desktop/tablet, it works fine. The issues are:

1. When viewing a product, I set the quantity (e.g. 5) for a product and click "Add to Cart". However, when I view the cart page, the quantity for that product is set to 1.

2. When viewing the cart page, if I change the quantity field for any product (e.g. from 1 to 5) and click the "update" button, the page refreshes but the quantity is still set to 1.

I want to stress that this issue ONLY occurs on mobile devices (both iPhone and Android), and is the same on every mobile device I have tested.

Here is the website in question: http://www.whatthepatientwants.com (OC v1.5.6)

I should also note that this is a custom theme, however all of the standard OpenCart scripts/functionality have been retained and not modified in any way. The only changes are aesthetic, and there are no vQmods which are affecting it (I have tried removing all and clearing cache).

Any suggestions would be greatly appreciated.

Sorry to bump, but I've spent many hours searching for a solution and haven't come across one. Does anyone else have this issue?

No one else has ever had this issue?