If the transmission is slightly misaligned with the bellhousing the clutch disc will drag the flywheel even with the pedal fully depressed. This causes the synchronizers to be difficult to move. Going to reverse will cause at least a slight grind, since the reverse gear is not synchronized. Did it shift OK before the new clutch was installed? Also, I hope you changed the pilot bushing in the end of the crankshaft. A worn pilot bushing will let the front of the transmission shaft wobble enough to set up a drag situation on the flywheel. It would be more likely to happen with a new disc with minimum clearance when released.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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