Add to Cart Button disappears

I have checked other posts about this and followed the recommendations:

1. Catalogue mode is OFF
2. There are no characteristics

I can see the add to cart button appear briefly then it disappears.

Any ideas about what else it could be?

When I change the quantity it does add the product to the store.

SOLVED:

When an item is added to the cart the add to cart button is replaced by the quantity selector.

The only weird behaviour is the quick appearance and disappearance of the button. It flashes on and off quickly.