Every now an then, usually at an inoppertune moment, the throttle or brake wouldn't work for me during a race. Even though I was pressing down on the screen with my thumb the throttle/brake pedal icons would dissappear.
I thought it might be hardware related so I did some racing on my wife's iPad. Same thing happened.
It didn't seem to be related to a particular car or track, although it did seem to happen more often at certain turns. In retrospect I think it wasn't related to certain turns or straights, it was only on more critical portions of the tracks that I tended to notice and remember the problem happening.
Figured it was a monkey sorta software problem and resigned myself with just living with it.
However, after some time I finally did notice a pattern. The common denominator turned out to be me. I found when the problem happened the base of my thumb was on the screen. Mostly that happened when I had tilted the iPad down quite a bit and/or when my thumb was mostly straightened out.