I've tried tightening it, I've tried adding more ptfe tape, I've tried having less ptfe tape, I've taken the thing out again to check and double-check the alignment of the joint . . . and it still continues to weep!

Very grateful for any ideas or suggestions!

Is it possible to get a stopcock that can be soldered onto copper piping, instead of compression joint? Only other thing I can think of is to remove the length of old pipe that it's on and start again with new pipe and olives (currently new olives on old copper pipe).
Cheers
Tony