MX-13 Engine Brake Stopped Working

So, I came out of a 2019 Cascadia and was put into a 2020 Peterbilt 579. At around 47,000 miles the engine brake suddenly stopped working on me. It was working fine the day before and the next day I turned the truck on and it magically wasn't working. Well here I am at 53 and a half thousand miles and it did the same thing again. The only difference is that I was driving my truck earlier in the day it was working fine and then I try to take off less than 2 hours after that and once again, the engine brake magically isn't working anymore. The last time this happened I had to bring it to a Peterbilt dealer and have them correct the issue which made me lose out on a day's worth of work. Does anybody have any idea what I can do to fix this issue myself?