Most reputable casino affiliate programs won’t withhold CPA amounts unless they’ve determined that some kind of player fraud was going on. I’ve heard stories of affiliates who’ve taken some programs for several thousand dollars by cheating the CPA system.
Your best course of action if you’ve already talked to the program directly is to post about the issue in that program’s forum here. That’s why we have those forums here: so affiliates and affiliate managers can solve problems. Your other course of action is to stick with the MGR and take comfort in knowing that you’ll probably make more money long term on MGR than you would with most program’s standard CPA offering.