I recently replaced the faucet in my kitchen sink and, after reconnecting all of the feeds, I noticed that there was a very slow leak where the cold water feed connects to the flexible tubing that goes to the cold water input on the faucet. I tightened that nut as best I could, but the problem persisted. So, I disconnected the hose and put some plumbers tape on the cold water feed. I reconnected the line, and the leak is still there. I have tightened the nut as much as I can without stripping the threads. Is there anything else I can do to try to stop the leak?

You may have accidentally cross threaded the connection. If that's the case they will not seat properly. The threads hold no pressure so Teflon Tape will do no good. Check your threads and try again. Good luck, Tom

Do you have rigid water connector or flexible?
In you situation, I would use flexible line. Its seal is soft so the line can be hand-tight. It is hard to cross-thread them.

I am using a flexible connector and I reseated the line several times last night with the same effect. The leak is extremely slow (a quarter sized puddle after 7 hours), but it's annoying me because it's the only blemish on an otherwise good job. I'm beginning to wander if maybe the rubber washer in the flexible line is bad. I bought it brand new, but it could have been defective. Do you think getting a new line might help?

I tightened that nut as best I could, Did you use a basin wrench,(see image) to get the best grip on the nut that you could? If not, then purchase one and try again. Good luck, Tom

I think he (or she) is at the other end of the supply line Tom. I suspect that the supply tube is pinched by the compression ring beyond it's ability to "spring back," or the compression washer is damaged.

Reissa, try cutting an inch off the supply line, (Plastic or copper?) if it is long enough, and using a new compression ring. Or just replace the supply line. (3-5 bucks.)