What you call "ramps" look more like wheel chocks. Your tires would get more purchase if you built up the ramp in layers, say with three 2x10s of decreasing length nailed or glued together like this (side view):
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Wait, does the tractor stall, or do the wheels spin? Or do the "ramps" just slide forward? There are different ways to fix this depending on what your specific failure mode is.
Steep angle with turf tires.... tricky. I would just saw off your ramps at a lower angle. This is really a pretty stupid problem, should be no issue at all.