Do you have any idea what I might be doing wrong?
Honestly,...no. Without watching you pick the lock it would be very difficult to tell, and even then it would be difficult to tell why you could only pick it once.
That said, whenever I have had this type of problem. It is almost always a phychological problem. Ergo, "I did it once, so it should be easier the second time."
The problem is that sometimes you only got lucky when you opened it the first time. Which won't help you be able to pick it a second time. Which is why when I practice picking, I try to avoid claiming to be able to pick a lock. Until I can pick it repeatedly, and consistently.
What I would do about the problem is practice practice and practice. And if you ever get frustrated, slowly step away from the lock and take a break. The odds of learning and figuring out the puzzling lock while frustrated are very slim.
Hope that helps and have fun picking
Farmerfreak