On Riff Raff this is also a PITA, very frustrating indeed.
I can only raise the mainsail with the wind dead ahead, which is by far not always possible.
I used a lubricant to make the slides go up easier, which helped slightly but only for the first part.
From my experience the problem seems to caused by 2 reasons:
a.) If the boom goes off center, the slides rotate sideways in the mast rail. Because of the profile of the rail there is a lot of friction on the slides and movement gets a lot harder.
b.) The top block is not positioned correctly (both length wise and sideways) above the top of the sail where the halyard connects. So the higher you raise your main, the more the top slide tilts both forward and side ways, effectively preventing manual hoisting.
For a boat this size it should not be necessary to raise the sail by force/winch, especially in light winds. It has kept me from sailing several times, I'm still looking for a solution.
Maybe this is also the problem on your boat.