Is there any spark gap adjustment on it? Air is thinner at higher altitude and thus less oxygen content. If the spark gap were a bit wider, it would ignite better. I once had the same problem with an RV furnace.
As a temporary fix, maybe you could use a long lighter (like for lighting campfires and fireplaces) to manually ignite the thing while it's trying to spark? I have no idea of there is access from the outside for such a procedure, but maybe it's worth a look.
(edit: whoops, I didn't see the part about it igniting but not staying lit. Yup, orifice size change might be the solution.)
