Hmm, New Diver is unable to dump air from a 55lbs back inflate BC. Is this a defective BC? Highly unlikely.
You are trapping air plain and simple. You need to become familiar with how to dump air with this BC. If you want to dump from the lower dump valve that needs to be the highest point. So stick your butte in the air, tilt up to the side where the valve is, and dump. If you want to shoulder dump that needs to be the highest point.
If your left side is the highest point and your dump is on the right this trapped air will expand as you ascend and you will end up with a run away ascent. Not good, not at all. Unless you have a stuck inflater, then You are the issue and you need to figure out how to use this equipment. You did not need 50lbs, or even 40lbs of bladder but that is what you got. Now you need to NOT ascend until you can dump. Once you start to dump then you need to ascend or you will sink.
You need to work on this until you can handle your BC without thinking about it. Yes it takes some time. Welcome to diving.