Speed
That Imperial Micromatic is a very good piece of engineering.
When you lift the platter you'll see two idler wheels. One is permanently set to just one speed and can't touch the platter, it is for the changer mechanism.
The other is supposed to move up and down against the stepped motor shaft to change speeds. The linkage to arm which moves it can slip out, dust and thickening lube can 'freeze' the post on which the control arm rests to one speed as well. I suggest you try gently shifting speeds while pushing/pulling very gently on the arm on which the wheel rides. If it moves easily and smoothly then springs back to one speed, then the problem is in the linkage, easy to just hook back up as one end will be hanging. If gentle pressure won't move it or it moves gratingly, then the post it rides on needs cleaning (and the resistance might have knocked the rod driving it loose).
Wonderful consoles, great turntables, Magnavox really built some good stuff.