Stark Arms G19 Trigger Problems

I just started to have a problem with my SA G19. I bought it used so that might be one problem... When I pull the trigger, it stays back and I have to take the slide off and manually push down this little black piece to make the trigger go back. Also the trigger will go back if I slide the slide forward once it's slid back. I know this gun used about %20 proprietary parts so it may make it hard to fix. Anyone know how to fix this?