That looks like a Pacific Trailer - at least from the detail I can see at the rear. Those are nice trailers, and from what I have seen (at least on tandems), they fit the boat nicely. They do run right to the back of the boat (which many others don't) and there is a part of the frame that thus extends pretty far down and is right at the rear of the trailer.
Here are the photos of your boat and trailer:
And here are a few other Pacific Trailers, that look like yours but are tandems. You can see how the frame extends well aft and down -- but with the tandems, the rear axle is further aft I think, and thus there is less frame overhang.
In contrast, my trailer (and other non-Pacifics I've seen) tend to have the frame stop short of the very back of the boat, and thus only the bunks extend all the way aft (and they are quite a bit higher off the ground than that framing member you have). Actually, I've always thought that was a nice feature of the Pacific (or any trailer that has a longer frame).
Here is a shot of my boat on the trailer just to compare. Pardon the ridiculous overexposure, but it does show how the boat sits on the trailer. As you can see, the trailer frame ends before the transom (the bunks continue on until just past the end of the transom, however). I would actually prefer that the boat sit a bit further forward on the trailer, but the way the winch post sits makes it hard to move just a short way forward.
So, I think perhaps I can visualize how it is scraping now. First you have the frame right aft and low due to the style of it, and then maybe because it is a single axle, it pivots a bit more in a see-saw fashion when you are going over dips or etc.? I'm not sure about that last, but just thinking out loud. As Chester says, is your hitch ball perhaps a bit high and so the trailer is angling down in addition to the other factors?
Since I can't see the trailer, and I don't know your tongue weight, this is just a thought: If you could use more tongue weight, then perhaps the axle could be moved aft a bit. I don't know that you could move it enough to make a difference though, since just a small shift changes tongue weight quite a bit. 15" wheels would help a little bit (and might give you better load capacity, depending on other factors).
Another idea would be to contact Pacific Trailers (who are in California). I called them to ask about some side guides (I really like those Pacific side guides, but they are very expensive to retrofit!), and they were great to talk to.
