Catching front edge 270 on

For some reason whenever I try and 270 on I seem to catch my front edge often, I've done them before on flat tubes and both this week and last week I tried them on a long box about a foot in width and I caught my front edge and went straight to my hip. Today I tried it again and the same thing happened except I was able to brace myself and sort of just hit my face on the side of it. Part of me thinks it might be because they aren't true flat boxes they are slightly sloped up but should that be an issue? Or should I just try them on down rails and boxes to avoid that issue all together.