To start with, do you even have cart visits?
If people land on your website, browse around but never add to cart, you’ll know something is wrong at the highest level.
A lot of other things can be part of the problem, and here are the usual ones:
* tracking not in place
* not the right keywords (or match type)
* competition too high
* targeting (geo/audience/etc.)
* running on the wrong network (in your case PMax could be spending all the money at the wrong place in the campaign with assets)
* Landing page issue (loading speed, UX, cart experience, payment options…)
* Product issue (pricing, appeal, offer…)
* something else