I suspect you will never get it flat without restretching in my opinion. You didn't mention whether there were cross braces on those heavy duty stretchers. I would recommend a cross brace every 30". Your canvas should have at least one on the long side.
The painting may also begin to show ripples in the canvas possibly from uneven tension while stretching which I'm guessing might be part of the issue. I have taken the warp out of some canvasses when they are framed. The frame can add more rigidity but not always. I'm guessing yours may not be framed.
I would restretch on stretchers with at least one cross brace, stretchers with keyed or mechanically adjustable corners.
Some of the nicer stretchers I have seen are from http://simonliuinc.com/
I have stretched a canvass that was 4' x 20' with cross braces using custom made stretchers with the mechanical keying system. They are awesome but not cheap. You get what you pay for.