If your problem is not corrected by the other suggestions, Unfortunately , you may find that your rear site has only the bare nub of the original sight adjustment teeth that are engaged by the sliding assembly. These teeth should be the same length as the top of the right side of the rear site assembly... Since these guns are very old, it is likely that the teeth on your rear sight assembly have been broken off by years of recoil force. (See the attached photo to see what the teeth are SUPPOSED to look like)
There is no quick fix for this malady... You can either replace the rear site with a reproduction, or you can have a craftsman of the highest skills weld over the broken teeth and then recut them...
My "Frankenartillery" Luger suffers from the same symptoms...
regards, and condolences
John Sabato
The answer to the photo question is HERE !
http://boards.rennlist.com/upload/LP-08Sightteeth.jpg