Re: More Weight
The limit may be based solely on the tires it came with. Have a close look at the axle and look for a weight rating on it. If it's rated for more weight than the trailer, replace the tires with higher capacity tires. You can also likely upgrade the springs to heavier duty units too. That said, in some states, you may be breaking the law by putting more weight on the trailer than it's rated for. In NH (where I live), the weight limit for light-duty (under 18,000 lbs, if memory serves) trailers is based solely on the tire capacity, not the capacity specified on the trailer tag.
If the axle is the weak link, not much you can do except replace it and hope the rest of the trailer is up to the task. Trailer axles are pretty cheap and easy to swap.