are you fanning the paper before putting in the trays?
is the paper correct for the printer? e.g. if too shiny might not pick up.
how old is the printer? e.g. if old, paper rollers might be worn (shiny)
You say it works sometimes, is that correct and if so, does it print ok?
When the tray is pushed in, does the paper position (maybe sprung upwards) against any feed rollers.
Would expect at least one, which pushes the paper into the first pinch rollers.
Good Luck - Keep us posted.