You've got a worn synchronizer. If it's just started this problem, just replacing the brass "blocker" rings will fix it. If it's already damaged the synchro section of the gears, then it gets a little more difficult, and expensive. As the shift collar tries to engage the gear, the blocker ring grips the end of the gear, and matches the speed of the two parts. It will take a full teardown ot the tranny to replace the rings, so you might as well put in a "small parts kit" with new needle bearings, etc., while you're in there. A slightly draggy clutch will accelerate blocker ring wear, so be sure you've got a good clutch adjustment.
Jerry
