Is it clutched or non clutched ?
Ensure sole plate rollers (if clutched) are free and also that the sole plate in ether instance is clean.
Hard to push i have had, but revealed to be customers carpet having a protective scotchguard sprayed on it and it left the soleplate sticky.
Heard of this issue, but never had it myself, did i read that in the past someone drilled a couple of holes in the soleplate and removed the soleplate string gasket (if fitted) ?