Sounds like there is something binding in the cocking/ hammer/ sear mechanism. The difference between forearm off & forearm on is the engagement or disengagement of the cocking hook on the barrels. With the forearm off the cocking hook is disengaged and the gun should open freely. When you install the forearm the un-hooking slide is slid back so the cocking hook can engage the cocking cam to cock the hammers when the gun is opened.
There is something either installed wrong or is binding up. I'd pull the trigger plate back off the gun and then give it an methodical inspection to determine what is binding the works up. Whatever you do, don't force anything, stop & backtrack until you find the issue. Some pictures with the trigger plate off might be helpful.
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