Two things could have happened. First, you may be using the wrong key. I have had some of the locks on my latches replaced after they failed or fell out and so now half my latch locks can be opened with my door key while the others cannot.
Second, these locks are really cheap and it could be something is wrong with the tumbler. Like I said I have had them fall out, but also rust up and other problems and generally don't trust them.
Come to think of it, the darn locks have been my one persistent headache with this trailer (small potatoes, I know). Anyway, I opt not to lock the latches anymore since I store at home.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD