HP-65 card reader repair - can't write card, but can read

[jay]

I've finally had the chance to play with the card reader again. I've found that it can read cards - I tried reading a card written in an HP 41, and it seems to read fine (no blinking, puts instructions in the program memory), though it of course can't execute the program.

I've tried carefully adjusting the eccentric cam a bit at a time, to no avail. What are the likely/possible causes of an HP-65 that reads cards, but doesn't write them?