First off, see if your execution is the problem or the speed in which you do the move. Get out of Trial mode and do training mode.
Her hcf+B after d.B, d.B, st.A is a pretty fast link. You have to do it pretty quickly. It's actually not very practical of a combo because it's so fast and doesn't had a considerable amount of damage, but it has pretty good corner carry.
The next one isn't very practical at all. The super isn't the problem. The problem is the d.A to d.C is a one frame link. It's really hard to time in general. Coupled with a qcf, hcb super move, you're gonna have problems.
A good combo with Mai to try and will get you pretty far is d.B, d.A, qcf+C or d.B, d.A, qcb+C. You can use EX moves and the like to extend combos, but try to keep it simple and just poke. Mai's big damage comes from corner damage.