10515 gear case problem

I have a 10515 Lawn Boy it is in excellent cosmetic condition I bought it from the original owner who said he used it less than ten times and then put it in storage because he didn't like the staggered deck. It runs great and the paint is almost flawless but for some reason it is hard to shift the mower from one gear to another I tried to move the yoke but even trying to move it by hand it is hard. once you force it into gear with the lever the self propel works fine but I know it shouldn't be this hard to shift. Does anyone have any suggestions before I open up the gear case assembly on what to look for? I'm positive it isn't the cable.