Advice on plumbing leak

[Donald S.]

I'm finishing up the plumbing for a new kitchen double sink. The two tail pipes converge at a T-pipe, which connects to the P-trap, then to a 90° elbow that connects to a short extension and into the wall. 

I cannot for the life of me get the p trap nut to stop leaking. Where the p trap connects to the 90° elbow. The best I can get is a very slight leak, but I know that's wrong. What can I do? I've tried using 4 wraps of plumbing tape, and even laid a washer in the pipe (the elbow doesn't have a washer and the piece is designed that way). What trick am I missing? 

Thanks

Cheers!