sink supply line leak fix question

hey guys!
fran here again.
a question in regards to a new sink install in my bathroom....
so i bought a new faucet online and it came with the flexible supply lines for both hot and cold.
consequently, the flex lines connect to the faucet itself and on the other end where it would connect to the water supply lines, it's the same 1/2" hex bolt end fitting as the grey pvc supply lines with 1/2" hex bolts, so i bought a couple of male to male brass compression fittings to connect the two.
when i went to lowes to get the brass "nipple" couplings, i talked to the guy in the plumbing department, and he said not to use teflon tape for the threads - that teflon would defeat the purpose of the rubber washers keeping leaks from happening.
so, i connected everything and it's slowly leaking.
my question is - is there any way i can fix the leak, or should i bring eveything back and just get a really long (like 30 inches) flex tubing to connect the faucet directly to the shut off valves?
the grey pvc supply lines running from the shut off valves have a 1/2" connector hex bolt thingy, and i think that's where the leak is happening. either the bolt thingy doesn't have a rubber washer, or the rubber washers have been compromised.
hard to explain without pictures, but i'll attach a couple of the pieces i have so you'll hopefully know what i'm talking about.
i know this is a unique connection situation - normally you just have the flex lines running from the faucet to the shut off valves, but i thought i could work with what i have.
any better way to do this?

With that setup, I would just buy long stainless steel flex lines or replace the grey PVC with stainless steel flex lines and use the nipples to connect the two sets. Messy but it will not leak.

by the way, not sure if i made this clear or not - it's leaking where the brass nipples are. nowhere else.
also, i know for certain that the brass nipple threads are the correct ones - i fitted a few at lowes before i bought them, double checked with the plumbing dept guy, and when i connected them at home, the fit was snug and no movement happened.
they do fit and are connected tightly....
hope this helps.

also, not sure if one flex line would work.
both the shut off valve and the faucet connection male parts are the same size - like 3/8" or around there.
do they make flex lines with both ends having the female receptors for 3/8"? if not, i'll be in the same boatr and will need to connect two flex lines together with a brass nipple, unless i'm totally wrong...

I bet the leak on the nipple is happening at the PVC end and not the stainless steel end, right? That is beacuse the flex lines have a washer that snugs up tight.

you are correct. the leak is either happening where the hex bolt surrounds the grey pvc line or where the brass coupler connects to the hex bolt (inside).
is it difficult to put a new washer in the grey pvc supply line?