I have an old Sharp VL-E33U Viewcam that won't retract the tape when a tape cassette is present after pushing it down. If no tape is present the mechanism slides in properly after pushing it down.
There is a clear plastic spindle that is on the mechanism that goes into a large round hole at the center of the tape cassette. It is possibly an optical sensor of some sort, not sure. But if I gently touch this spindle with no tape present the mechanism will not retract. If the spindle is not touched the mechanism retracts with no tape. Also, I tried placing a small black and then a small white object over the spindle and they also prevent the mechanism from retracting. They are not physically blocking anything but just cause the "sensor" to stop the tape drive mechanism from sliding into the play position.
Also, the small backup ni-cad battery is dead. I don't know if it could possible have an effect on this sensor or viewcam operation. The manual implies it is only for time/date and other settings backup.