[RobM]

Personally?
I say buy something harsher. I've had the same problem with a spanking partners (I won't say 'spankee's regarding them in these cases since they're far older and more experianced than me) before and I've found, go for the gold.
Purchase a leather strap or a tawse (Practise to get the hang of striking just right) and that'll probably make them sit up and take notice. Small straps can still be used in a limited way otk with practise and they still do nice alpha.
You may just have reached the limits of an ordinary hairbrush. Consider getting a silver backed one as well.

A wet bottom is also good advice, I don't know if it's practical for like, twenty minutes of spanking - but try it and see, it's a good idea.
You could consider distractions to lower her tolerance such as tying her wrists or hands, having her right over the edge of your lap so she has to hold herself from falling without support or something.

Oh, and consider going on for longer than twenty minutes, thats around about the most I've ever spanked for, probably longer, but I've heard of spankings going on for over an hour!
Thats gotta burn like 600 calories!