If you have a bunk on the trailer that is not parallel with the other, the boat will have a hard time centering. I have been loading different boats for years, and never had a problem until I bought a new boat last year.
I struggled loading that dang thing. No matter how i loaded it, or backed the trailer down the ramp the dang thing always wanted to move to the right. So much so that it would sometimes almost miss the front roller.
I never in my life had this much trouble loading a boat before...........was it me? was it the boat? It turns out that my brand new boat came with a brand new trailer..............It turns out that the bunks on the trailer were not parallel. The left one was over an inch narrower at the front thereby pushing the bow to the right every time. I re-centered the bunks and waa laa problem solved.