Whats the problem?

My 62 Valiant has a slant 6 and a 3 speed manual transmission and when I'm driving its hard to get it out of first. When I try it while parked it shifts freely, . I've tried shifting while holding the rpm up, matching rpm to road speed,letting rpm drop before trying, adjusting clutch pedal free play up-down, but nothing seems to work. Oil has been changed in it and that didn't help either.
Help!! :prayer: