Walk Behind Lawn Mowers

I recently purchase model HU725AWDHQ lawn mower from Lowe's. Used it a handful of times. During my mow yesterday about halfway through, when I engaged the drive it started running rough. I didn't hit anything so don't think it's the blade, particularly since when I just push the lawn mower without engaging the drive, it cuts just like normal, without any vibration or roughness. I have cleaned out all clumps and such, tightened the drive cable, roughness still there. The drive does still function, just rough. What do I need to look for or change out?