You didn't say if you were quilting or piecing. That weight thread wouldn't normally be used for piecing quilting cotton, but quilting with it is okay. You didn't say how the tension was either. Perhaps the other C +C thread will be fine. I bet it is also 50 wt. I'd try the others before giving up with them.
i have also heard how different colors can affect the thread as well. Something to do with the amount of dye required can make it thicker. That could affect tension, so why not cause other problems.