Phaser 5550 will only print from one tray

Hello,

My Phaser 5550 used to work beautifully, but now it always prints from tray 3 (out of 5) and won't move on to another tray when this tray runs out. All the tray selections are set to Automatic. I have tried changing the tray order (starting with 2 or 5), but this makes no difference: it only lifts paper from tray 3.

Any advice welcome.

Many thanks.