Can't modify the thickness of a simple part. It is grayed out!

Thanks for any hints for solving this efficiency issue.

Thanks for any hints for solving this efficiency issue.
I revisited the problem by duplicating again the assembly and the part, and found out the culprit is a relation created in the assembly. This relation is not shown in the part mode, but in the assembly mode. It turned out that the grayed out thickness is set to be proportional to a thickness of some other part in the assembly. (The batch of relations were created by myself one year ago).
Thanks for the hints! Especially the one to check the "message at the bottom of the screen" (and "check Message area -" from Martin).
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.