Technics SL-BD20D Service Manual

I have a Technics SL-BD20. I'm experiencing an issue where, when playing a 7" 45 rpm record, the tonearm (if it even lowers and the stylus makes contact with the record) almost immediately gets automatically returned to the tonearm rest. When playing a 12" 33 1/3 rpm record, the tonearm reaches about the same point that would be halfway through the grooves of a 7" record and then starts skipping. The tonearm does not pick up and return to the tonearm rest; it just stays in that position and skips. I tried adjusting the screw near the tonearm pivot from the top, but that didn’t fix the problem. I opened the turntable and disassembled it enough to check for any obvious broken or misaligned parts. I noticed that the round white plastic disc with a +-shaped offset hole was placed over the small opening, which seems to be the access point for adjusting the auto-return position. However, it was just sitting over the hole without any obvious way of securing it. How do I reassemble the turntable so that this disc is properly held in place? I can’t figure out how to do this from the schematics in the manual.