I have a 1986 Fiero 4 speed manual and I have replaced the clutch and slave cylinder and it still wont shift what could the issue be?

I remember Fieros having clutch problems before they went out of production and that limited the availability of good clutch cores, the way to do it is a clutch kit that comes with clutch disc pressure plate flywheel and throw out bearing, after that it's a matter of careful assembly hope this helps