A kfb (knit twice into one st) is done in a stitch, the M1 is between sts so it can throw off your incs if you sub it. However you can kfb in the first st in place of sl1, k1; unless it's an outside edge st, you don't need to slip it. If it's going to be seamed, a slipped st edge can be more difficult to do. There are a couple different M1s on the increases page, you could try another one to see if it's easier. Where are you doing the incs, along the seams, neck edge or what?