Hi!
Taking a guess here based on the HP LJII/LJIII line of printers...these units had a set of three metal strips, one of which would be pushed in by a plastic protrusion at the front of the paper tray. If memory serves, this was at the lower left of the paper cassette as you'd hold it when putting it into the printer.
The type of paper tray in the printer would be determined by which of the three switches was pushed down.
I would tend to think that you have a bad switch, something blocking the switch, a damaged paper tray, or perhaps even the wrong paper tray.
William