Leaking valve

Dear all,

We've got a leaking shut-off valve (I think that's what they're called) so I took it apart and it had the old hemp stuff at the base that formed a seal. I used some teflon tape folded over itself and wrapped it around about 10 times and then tightened the nut. Apparently no leak. Then when I checked a few hours later, there was a small leak. I tightened the nut further and no sign of a leak but 24 hours later, it's a little damp around the base of the valve. Question is can you just add more tape to what's there or do you start again or just leave it?