Hello,
Are you saying that the record will not reject when playing the A side?
If you lift the arm and move it towards the centre of the record can you hear the read switch operate, it should do because from what you say it rejects the B sides.
When you play the A side and press the reject button does it reject?
Have a look at the cam switches on the side of the mechanism, check that none of the uninsulated spade connections are touching each other (shorting out the switch).
The problem could be as above or could be a cam switch not ajusted correctly or there could be a problem with the Mechanism Control Unit.
Regards
Alan Hood
ami-man
UK