I would check the switch associated with the disk in the center of the platter as that has a switch that tells the system a record is on the platter. If it is intermittent (dirty/misadjusted) you will get this behavior.
Not necessarily related but the default speed is 45rpm on both 7″ and 12″ records. This can be changed by pressing the 33 button but normally I would expect a 12″ record to automatically run at 33.
I would check that the Disc Indicator switch under the center 45 adapter is making good contact first. If it is opening and closing properly, then your problem is in the logic that it drives which is TR1 and Tr2. There are voltages in the service manual for these.