No, I haven't had any problems with my CS4000. A while back, I lent it to a friend and he's been using it off and on for 6-9mos.
1) Current NikonScan software loaded correctly, etc.?
2) Scanner has latest firmware? I know some versions of NikonScan require updated firmware.
3) Does the slide adapter work correctly? If the slide adapter works correctly, then your SA-21 might be bad.
I'm assuming the slide adapter is using he same motor, mechanical parts in the scanner to load the slide.
I would suggest taking you adapter to a local dealer that sells the Nikons canners and trying it in their demo.
We have a local shop here that has all the scanners on display. They will usually let the "regulars" hook their laptops up to test/try them out.
Hope this helps.