I encounter the same problem from time to time. I've found the most common cause with my trailer is one or more of the stab jacks being too tight - causing the trailer frame to twist, which misaligns the door. Before dumping my tanks at my seasonal site, I like to raise the jacks an inch or so, so I can rock the trailer before dumping. (I think it helps stir things up for better dumping.) It's not uncommon for me to get one a little tight when I extend them down, again. Sometimes, just "tweaking" them takes care of the latching problem. I've also found spraying a little lube (pick your favorite one) on the latch, strike plate, and hinges about every 10 - 14 days takes care of the problem. I also found out the hinges on my screen door had loosened, causing my screen door to stick closed. Simply tightening the hinge screws took care of that problem. If all else fails, follow 3'senough's advise. Take a look at both the strike plate and the latch. The wear pattern on each will tell you how they are aligning. It's a rare strike plate that doesn't need a little adjusting from time to time - even in your stationary "stick'n'bricks"!
Good luck finding a solution to this "sticky" problem!