Are you jointing and planing your wood to a particular spec, or using big box wood? There is no such thing as straight, flat bigbox wood, so it could just be variations in the material's thickness and it not being straight. That bit brand is OK, not great. My default is Freud (available), but for anything exacting, like doors and T&G, my personal favorite is Sommerfeld Tools, which have matched sets and their wonderful EasySet setup jig, which accounts for exact stock thickness. Matched sets means you don't have to reset the second bit's height (you drop a half inch wide rubber grommet in the collet to form a reliable base). I LOVE the EasySet jig, and Sommerfeld has one for Freud bits as well.
The more I do, the less I accomplish.