Collision problem

[Platypus5]

I am working on a project that involves a "grabber arm." It is an ordinary type of claw, you know, two big "fingers" opposite of each other. However, for some reason, one of the fingers will not detect collision properly. Objects will go through the arm. I have checked the settings, and the group for this finger is exactly the same as the settings for the other finger group, which does work. No ignore buttons have been set.