If you are using a steep ramp the bow might actually be hitting the cross members anywhere on the trailer, even between the bunks. You can move the bunks in some, buy some rub pads for the center of the cross members, or just get another trailer. I would try moving the bunks inboard some if you can, and if you feel comfortable with it.

Edit: I just looked at your pictures again. You defintely can't move the bunks without some major work and adding more stuff to the trailer, and I see you already have some bump guards on the cross members. Your only two choices look to be raising the bunks, or buying a new trailer.