I have a double door front entrance. The second door(the one not frequently used) does not easily open because it catches in the groove for the vertical lock. I usually need to unhinge it with a flathead screwdriver. I've tried to lower the strip at the bottom of the frame but it still catches! I've attached a picture. Any ideas on how to fix this? Thanks!
Assuming the door is positioned in the frame properly and the stopper is all the way up and stays up then you should be able to grind a bit off of the stopper. If you don't have a handheld grinder you can get a grinding stone that will work with an electric drill. The stopper is probably hardened and will not grind quickly but it doesn't look like you need to take much off.
It looks to me like the door is set poorly or has sagged, resulting in the bottom being too tight to the threshold. The adjustable threshold appears to be bottomed out at the point where the doors come together, and is higher in the foreground.
Rather than modifying a mechanism that should work fine, I'd examine the door install for square, level, plumb, and swing gap. Use long screws through the hinges to make small adjustments. If you post more photos I can offer more help.