the cover over the coils

[kramlavud]

I'm stumped; I removed the 6 long bolts holding the cover over the coils on my 94 E320 to change the spark plugs today. I have done this several times during my ownership of this fine car. After I changed all of the plugs, I put the cover on and insert the 6 bolts. I can tighten just 4, but 2 jsut spin, as if the threads are stripped. So I remover the cover, and put the 2 bolts into the threaded holes and gently turn, and the threads catch. But in serveral more tries with the cover in palce the bolts don't engage the threads. I can't find any interference that would keep the bolts up and away from the treads in the hole.
Now the car runs fine with new spark plugs. but I have 2 bolts just sitting in the holes without any engagement of the threads.
Any ideas?
regards,
Mark