Ok, (I got it backwards - No bike to look at right now), so that's good that if fits in the low position.
Sounds like the front edge of the seat is hitting the tank and preventing it from going down enough to catch in the latch. Two things you can try, one is loosening the tank pivot bolt, (its got a rubber bushing), and the front side mount bolts for the tank. Then see if the seat will latch in the high position. If it does, tighten the front side tank mount bolts while the seat is locked down in the high position. Then remove the seat and re-tighten the rear tank pivot bolt. It should still allow you to now latch the seat in the high position. This just re-positions the tank a tad farther forward.
The second thing is what Cycledude suggested. There is only one metal bracket on the bottom of the seat. And yes, it's the one with the loop that fits into the latch. Take it off and put a washer under it so it sticks out farther from the seat pan. That way it is effectively lower into the latch and should catch and lock.
Your wobbly description makes me think that it's more about the tank interference than the latch.