A photo would really help.
There's no issue with on-point sashing, and it does not need to be cut on the bias.
My best guess is that your blocks are not perfectly lined up so that, when viewing the entire quilt, the sashing looks not straight. Did you use cornerstones for the sashing? This is one way to keep the blocks lined up.
If you did not use cornerstones, did you mark the wrong side of the long sashing pieces and pin before sewing them together? If not, your blocks are probably not aligned perfectly with each other. I have posted before about how to do this. If you need those instructions, I can post them again in this thread.