The "Add to cart" button doesn’t respond on PrestaShop

What actually happens on the click

On a product page or a listing, the "Add to cart" button fires an Ajax call to the theme’s cart controller, without reloading the page. If nothing visibly happens, the problem is almost always upstream of that call: a JavaScript conflict is stopping the button’s script from running at all.

The most common case is jQuery loading twice, once from the theme and once from a module that adds its own script without checking what’s already loaded. The second load overwrites the first, and every event handler attached before that point stops working, including the cart button’s.

When the click triggers a 500 error

If the Ajax call does fire but comes back with a 500 error, the cause is almost always server-side, in a module hooked into an action triggered when adding to cart — actionCartSave, for example. A module that fails silently at that point blocks the action for every visitor, even though the rest of the site works normally.

Another common case: a required combination (size, colour) left unselected. If the theme doesn’t validate this properly on the client side, the add fails with no visible message, which looks exactly like a dead button.

Configuration or development?

Clearing the cache, disabling modules one by one to isolate the JavaScript conflict, or checking the error logs to spot the faulty module: this diagnostic work is often enough to pin down the exact cause. Fixing it becomes development when the conflicting module’s code needs rewriting, a hook has to be adapted for the installed version, or checkout needs securing against double submissions.