Couple of tips, and one way to test
First, tips - get it off the floor and don’t put it under your bed. This increases airflow and removes that as a possible problem.
You can also try setting the AC even colder, although 68 should be low enough that it can get to 60 or so at least
Now, how to test if its functioning correctly - set it to low when you aren’t in the bed and let it run for an hour or so - see how cold it gets. Bonus points - set the AC to what it will be when you’re sleeping to fully recreate the ambient air conditions
If it can’t get to 55 in those conditions you can (probably) exchange it for a replacement unit. If it can, then, most likely, you are the problem. Your body heat is overwhelming the unit and it will only be able to get so cold. In this case, if you want it colder, you’ll have to try less blankets/sheets (i.e. better airflow over the pad itself)