Hi Did you fit a new pilot bearing in the flywheel, if so did you put it on the shaft to check fit first. Wouldn"t be the first time i was given a wrong new bearing with a smaller internal bore/ Have had one too big as well in the past but found them both before assembly.!
Other thing is did you use an alignment tool to center both discs to the pilot bearing in the flywheel.if not that might be slightly out of whack. You could have enough play in the shafts they both went through. but are a few mm out for the input to go in the bearing. have done many tractor clutches some go easily others are a P.I.T.A. Have you slacked back the clutch rod adjustment also. could be the release bearing is hitting the clutch toggles stopping it going up. if release bearing is adjusted right forward, or clutch levers are to far back are they set up to factory specs for distance.
I Have had some go back together that needed a gentle even pull with bolts to close them up. But using this method you need lots of experience doing clutches to gauge whats enough pressure or to much without bending discs.on a case tractor a guy can get a disc the wrong way round and that won't let it go the last inch to, don't know if that can happen on a massey or not. As with all internet diagnoses if I was there I"d have a better idea on if you got a problem or not and your just slightly scared of messing something up. There are just so many variables on every clutch job