I havn't asked you the model of this lift yet, is this a tractor? or an actual forklift?
i've been assuming it was the model shown at the top of this thread, i've just been basing my comments on how generally a carriage/roller setup usually is configured on any lift truck, so can you give me the model? and i may can find something on it, maybe
but with that said....

when you have the carriage out and put the rollers on the stub shafts they are slightly loose?
They should be a positive slip fit i would think. (no wobble after they slide on the stub shaft).
And they should turn freely no rubbing when slid up all the way on the shaft. No retainers for the rollers?, ok i got that part which is ok, that was the old way the mfg's did it years and years ago. If they do not fit this way then either the bushings in the rollers are worn out or the stub shafts are worn. Either way they should not be loose as in wobbling, they should slide up on the shafts snugly but still turn freely.
And as for the shims/washers between the roller and stub shaft, those should be there to set the side play gap between the rollers and the sides of the channels so the carriage will not be loose and rock side to side too much. You may have to experiment with those to find the right thickness to set, it may only require one on each roller if only to just keep the roller from contacting the back side.
I suppose if those rollers are too loose on the shafts they could be binding up after the carriage is installed back in the channel. Without me being there to actually see what is going on i'm just giving you my educated guess as to whats it sounds like is going on.
It's just been my experience that the carriage rollers should fit snugly on the stub shaft but still roll freely, before reinstalling the carriage and then after installing it in the channel.