hopefully it is as simple as something like what robert said.

i rebuilt mine and it was fine for a year or more, and then became harder and harder to get into gear with the car at a stop. became almost impossible.

in the end i found that the shoulder on the input shaft was slightly touching the bearing (bearing, not bushing), in the crank, and my bell housing face squareness was about .015, where the FSM calls for about .008 max.

i fixed those two problems, and it mostly went away. still happens once in a while, so i know i most likely didn't find the real problem, but these two were definitely contributing.

has this just started happening? was it running ok for a period of time after the rebuild?

and does it grind going into reverse? that signals that the input shaft is still turning, and so the clutch not fully releasing. something to check out.

not the funnest thing in the world, but take off the inspection cover and crawl underneath with it running. have someone push in the clutch, and you should see the disc come to a complete stop. otherwise its not fully releasing.

get all the simple adjustments out of the way first...