Cannot attach mesh to skeletal mesh anymore

I was able to attach a tool to a person. But now skeletal meshes don’t appear anymore in this attachment list.
Does anyone know what could be wrong?

Alright, I solved it.
Both items need to have sockets; no attaching if one item is missing sockets.

Then again another time it doesn’t work for mysterious reasons.
I have 2 different hats and constructed them the same way with the same socket.
One can be attached to a guy, the other one cannot. So strange…

Alright, I solved this. The mesh has to be set to “movable” first. Makes sense.