I`d whip plate out, check if that was fitted correctly , wrong way is possible
wire brush input shaft, little wd,
then wipe dry, very small amount of copper grease, get friction plate gliding on input,,
wipe any excess grease before fitting
then check plate is fitted as it should be flywheel side to flywheel,
make sure your thrust bearing is gliding on the tube freely too, little normal grease ok
chuck box in , put some bolts on bellhousing and the starter in and test, should be fine ,
least you won`t be guessing what it is, you will have removed a lot of "what ifs"
chances are,, it`ll be fixed