Just curious, did this start happening after driving for a bit, or the whole time while driving? I would check the fluid in the manual trans to see if it's low on gear oil. Mine was real low on mine when I first drove my truck home, and the syncro's, gears, etc got hot, and could not get it out of first gear, until the trans was cold, the next morning I went out to the truck, and it shifted fine.
Then again, it could also be shift linkage, clutch, or clutch needs adjustment.
I've also had problems with the Hurst indy floor 3 speed shifter, it would hang up, but would lock the trans with the rear end.
I fixed this with new bushings, and tightened the base of the shift kit assembly.
David