Some trailers have a beavertail back end which lowers the floor at the rear and consequently lowers your ramp.  Apparently yours does not, nor does mine.  The beavertail models are a tradeoff though as you loose about 2' of usable floorspace where it angles down at the back.  I do not have a big motorcycle, just a scooter and atv but I understand exactly what you are saying.  If there is an easy fix other than extending the ramp with standoffs as Protector suggested, I am not sure what it would be.  Good luck.