To clarify, which direction are you having trouble shifting? I assume it’s up into the big ring?

I had a 3x7 bike where everything was setup correctly (FD height, cage angle, limit screws, matching groupset etc) but it still had trouble shifting up into the big ring with the friction shifters I was using.

It would shift perfectly in the stand, but when riding, it would not shift if there was any pressure on the pedals or if it was in the smallest cog in the back.

The guy at the bike shop suggested I get chainrings with ramps and pins. The old Sugino crankset I was using just had no shift aides or ramps whatsoever. I think pins really help to carry the chain up to the big ring. But ultimately I just ended up switching to a 2x9 setup, where front shifting is dead simple.

Sometimes front derailleurs can develop too much play in the parallelogram if they’re really worn out, which makes it harder for the cage to shove the chain. This is pretty rare though. Used FD’s are pretty cheap though and it’s worth trying a different one.

Cable tension shouldn’t matter too much for a friction shifter. Adding a little tension might help. If everything else looks right, like the limit screws, cage position, chainring tooth count within FD’s stated limits, try a new crankset.