open circuit glow plug engine code

My '05 jetta tdi 1.9; has a been throwing a "glow plug open circuit" on cylndr. #2, code lately. I replaced the glow plugs and re-set/erased the active code.
However every couple weeks the check engine light returns and the same code is tripped, whats up with that?

Did you measure the resistance of the "bad" glow plug, and compared to the others?

You might have a bad glow plug harness, or just a bad connection in it.

What is the fault code it's throwing?

Try some DeOxit on the harness.