Sounds like your lock up on the trigger guard is loose. The good news is it's pretty easy to adjust. The bad news is you have to remove the forearm to do it.
1. With the forearm removed and the Sub open and locked look for the round barrel nut against the receiver.
2. Tighten the barrel nut by HAND ONLY so it is hand tight against the receiver. Test the tension by releasing the trigger guard and folding and opening the barrel. The trigger guard should JUST seat as the barrel is reaching full extension. Any sooner and you may be loose, any tighter and the trigger guard either may not lock or you may have trouble releasing it. You can use a towel or something similar to hold the nut, BUT DO NOT TIGHTEN IT WITH A TOOL.
3. While keeping the barrel nut from moving, re-install the forearm. The forearm is what holds tension on the barrel nut and prevents it from loosening.
Let us know if this helped or not.