compression valve leak

trying to plumb in a new bathroom sink and have successfully sorted out previous mishmash of waste pipework and removed old corroded compression iso valves. However I am having a big problem with the new iso valves as they wont stop "sweat" leaking. I have used PTFE and plumbers mait which seem to have sorted a couple of the joints, but not all. Was advised the olive might need an extra turn or two, but every time I tighten a joint it either doesn't work or seems to start one of the others off again.
These pipes are going to be at the back of a built in vanity unit and so they have to be totally leak free.
Spent most of today trying to solve this and am fed up running up and down stairs turning mains on and off !
I hope someone out there can help with this. thanks

I assume you are connecting to copper pipe. Is it clean and paint-free?

yes it's old copper pipe and new iso valves and i cleaned the old paint off. About to go back now and redo the leaky joints with ptfe on the olives, hope for better results. Worried now tho that I probably overtightened everything yesterday with my constant nipping and if so what do I do next.

Use a pipe sealant if you get no joy with ptfe or remove olives,clean again and fit new olives with a smear of sealant on the olives.Copper olives tighten easier than brass

decided to stop faffing about with this - bought some Fernox SL-X. Problem solved, zero leaks .
This is one very happy DIY plumber. I will never do plumbing again without this stuff. .

Thanks to the people who replied to my post.