PTFE is not made for use on compression fittings. It never has been and
never will be.

However, as with most diy offerings a resourceful person will read that
it can be "used" on compression fittings but it is primarily
geared(IMHO) for making a watertight seal on a thread/nut combination
i.e where water can squeeze around the threads due to the space between
nut and thread - otherwise you wouldn't be able to screw the nut on!!!
The watertight seal in a compression fitting is made by the olive
(brass ring) compressing slightly onto the pipe whilst at the same time
being squeezed up tightly into the neck of the fitting. The subsequent
mating of surfaces is watertight. Just try to pull an olive off a brass
pipe once a joint has been 'done up' properly. In most cases you have
to cut the pipe again.

My old man made them for 30 years and before speedfit/pushfit plumbing
came along they were the godsend for DIY plumbers who didn't want to
solder fittings. Yes they are a bit more wieldy and do take a bit more
time but they give you the opportunity to break the joints and remake
where a solder joint is usually well fixed for life!

Compression fittings do leak - ONLY WHEN you haven't followed the
correct procedures or you have a faulty olive or fitting.

Some of the more common issues -

Don't reuse olives - they cost a few pence and if you have already
squeezed them onto one pipe whilst doing up a joint, chances are the
next pipe you stick it onto will be a slightly different shape.

Make sure the copper pipe enters the fitting in a straight line. If it
goes in squint then you will have to tighten the fitting more and more
until you get to waterfastness. If you tighten it too much you will
start to compress the pipe and leaks will develop. A good rule is to
get it to a tightness that feels comfortable ( 10 inch spanner ) then
roughly three quarter turn from then. If you get leaks, then notch it
up a quarter turn each time until the leak stops.

Make sure no swarf ( little bits of brass from cutting the pipe) get
into the joint, either under the olive, or into the compression fitting
itself i.e clean the pipe before you make the joint.

Don't use pipes covered in paint or other crap as they will affect the
waterfastness. Either sandpaper off the paint etc where the joint is to
be made or use a new piece of copper.

If you use plastic pipes then make sure you use a small pipe insert at
the compression fitting. Yes you can make the joint watertight without
it but given you are compressing plastic pipe then it doesn't take much
to screw it up. The internal inserts costs a few pence and give the
pipe some rigidity at the olive point.

Simplest way is to make all your joints as tight as you thing they
should be( and you can make them reasonably tight) then turn on the
water...Dry all joints with some tissue paper and wait a couple of
minutes..Then go back over with a tissue and see if it gets wet. If no
visible drips but paper is a little wet then that joint needs a
tweak...If you have water p*ssing out of the joint then you've either
not made it tight enough or you are missing an olive!! Yes it has
happened now and again....

Good luck...