Sorry to hear of your troubles. Sometimes steam engines
get cranky if they aren't run now and then. Does the
steam pipe have an oiler? Is it a slide-valve engine or
an oscillator? Sometimes, the piston/cylinder need a bit
of steam oil on the walls to help with the seal. Not too
difficult for either type of engine.
On the other hand, is the cylinder binding up? In that case
you will need to figure a way to get some penetrating oil in
there to dissolve any congealed badness. Pretty easy.
Please show us a couple pix of your engine for our enjoyment.
Good Luck,
Wayne