Raer Hatch won't close

The rear hatch on my 1980 928S (Euro) won't stay closed.  It seems to be a pretty simple mechanism but so far I haven't been able to fix it. I assume it is the plastic internal piece that is worn although it doesn't seem to be worn very much.     I ordered a new plastic piece which was supposed to fit, but it's designed differently and won't work.  Maybe the early 928s's were different.  No alarm system. Any tricks of the trade or souse for the right part?

Thanks.
Mike Lemons

I'm not sure how helpful this is but, I had a similar issue with my '84. I noticed when unlocking the hatch with the key, the plastic notch wouldn't return to place. It would stay in the open position and the hatch wouldn't remain shut. Wiggling the key back and forth when the hatch was open would release the plastic notch and the hatch would close and keep shut. Not an ultimate solution but a work around. I assume some lubrication is all that's needed. 

Clyde