Probably lack of oil if nothing else, is it sparking and chain stretching a lot?
Most likely if one side is sharpened slightly different to the other but then you say it's new, unless it got blunted on whatever wood you're cutting because of high mineral content.
The plates and bar where it is clamped should be clean, the oiling hole cleared out and the channel needs to be cleaned out, sometimes it can seem clean but a little stone can stay lodged and mess everything up.
Basically once it starts to curve at all in the cut it is tempting to carry on but you're better off taking it out and making it cut right, starting a new cut if necessary. Seems time consuming but often it saves more time to start off again, having a spare bar and a few spare chains becomes the best solution.