Anytime you use plastic for water lines regarding your situation, ALWAYS use the plastic compression ferrule along with the brass stiffener.
When you do this, thread that compression nut on and hand tighten it. THEN, turn the water on, it will leak. Take a wrench and turn it only until the leak stops.....then give it another 1/2 a turn and you're done.
Overtightening can cause a leak as well as using a brass ferrule on plastic small tubing.
If there is any bend to that tubing before it enters that compression fitting, it will slowly cut into that pipe and cause problems with a leak.
That happens a great deal in my area with plumbers using pex toilet and faucet supplies and use the brass ferrules that came with the angle or straight stops; they cut the pipe eventually which is spelled danger in many ways.