Funny, I noticed the same thing on my SX-A a couple years ago and still haven't figured it out. I've looked around for obstructions on the transom but there are none. I didn't see anything noticeable in the engine compartment either, but since the steering system is a little foreign to me (and I have a tendency to cause myself headaches by taking things apart when I shouldn't), I decided to leave it alone.
If I remember correctly, turning port feels like it hits some sort of stop. It is a sudden firm stop and will not turn port anymore. It turns more starboard, but that direction is sort of a mushy stop. About the same time I noticed that my steering wheel alignment changed. Straight ahead means having the wheel caulked about 30 degrees to one side. I tried adjusting the wheel but no help.
I'm not sure if something like this can suddenly happen, and what causes it, or if it was always that way and I didn't notice before. Hopefully you find something that I missed.