I know this is going to sound like a “Is the computer turned on?” sort of question, but have you opened the mesh in the mesh editor and verified that there is an actual “collision mesh” on it?
I’ve fought with this type of problem once or twice only to realize that there actually WASN’T a collision mesh on something that I had assumed that there was. So, sorry if the question seems obvious, but you do understand, I HAVE to ask.