Another possibility is a problem caused by the Tormat Write-In, Read-Out, or Trip circuits. To troubleshoot this, first, check the Write-In voltage (usually labeled 'Write-In Source' on the control center or selection receiver test terminal strip). For the digital machines (black/gray or red boxes), the voltage measured at this point will be about +125 VDC. For the solid-state machines, this voltage will be +395 VDC or +150 VDC depending upon model and number of selections. For the tube-type machines, this voltage will be -300 VDC. The Techie Stuff section of this website talks about how the Tormat works, so you should check it out before proceeding. If you measure significantly less than the correct voltage, check the Write/Read supplies in the control center or selection receiver. Given the symptom, it is probably reasonable to assume that the other supplies are functioning properly, but it doesn't hurt to check them, too. There should be test terminals for these supplies on the same terminal strip.
Disconnect the RCA plug sense loop connector from control center or selection receiver. Connect the negative side of a C or D cell battery to the center pin of the plug, while grounding the positive side of the battery to set all toroids to the selected state. (Note that if you use this procedure on a tube-type jukebox, you must reverse the battery connections.) Plug the sense loop connector back in and manually scan the mechanism. It should now trip at each record, transfer, and play it. Upon reject, the mechanism will move to the next record, and repeat the cycle. If this does not occur, the problem is in the Read-Out or Trip Circuits. If operation is normal, the problem is in the Write-In circuits, which will need repairing.