I had a bathtub that drank water like a sieve. I had a blockage and have had the line power rodded three times and it still backs up. Now the water has drained out of the tub but there is about an inch of water lying in the pipe. The plumber I had out says it could be a main line but all the other pipes in the house dispose of the water promptly.

I need more details.
I've have had the line power rodded three times and it still backs up. Where was it snaked from? The roof vent or the tub waste and overflow?

There is about an inch of water lying in the pipe
What pipe would that be? The drain?

The plumber I had out says it could be a main line.
What pipes are you and the plumber talking about?
Since all the drainage for the tub is under the floor line where are all these pipes you're talking about? Please explain because I'm in the dark. Can you shed some light on all this?" Regards, Tom

What does this mean "drank water like a sieve" How are you seeeing water inside the pipe? More details please