All four pins accurately hold the cassette level at a specified height. In the rear, center of the cassette, another pin goes up to release the cassette's internal brakes. A significant amount of spring tension remains on the cassette as long as it is loaded to firmly hold it down on the gaging pins. You should be able to press down on the cassette with your hand and feel no movement. Power supply defects - If the VCRs power supply is defective, the carriage can't get the electricity it needs to operate.
Foreign objects caught in the works - small kids are famous for putting things in VCRs.
Even if you don't have kids, be aware of the fact that even a label coming off a cassette is more than enough to stop a carriage dead in it's tracks. To check for mechanical defects, try to hand cycle the carriage and see if it will go from full up to full down and then back up again. You will have to release the tray latches before it will go very far. Carefully examine it's travel, checking for gears or levers that may be binding. Also check to see if the tray is derailing somewhere.