I have had dart frogs hide for 7-8 months. They may have been coming out at night or when I was not around but I did not see them. I had a tank with 3 green leg Panguana sirensis that I ended up giving to a friend of mine because I thought it was empty. He put it in the corner of his frog room for a couple more months. when he went to clean it out he found 2 of the 3 frogs hiding in the back ground. I also had a group of 4 orange galacts. I would regularly see 2 and occassionally the a 3rd. I was convinced the 4th had perished. When I eventually sold the "trio" I found the 4th one that was missing about a week later.
Kevin is correct about humidity and comfort in the tank. Your frogs will generally be willing to venture out in the open when they feel they have a secure place nearby to run for cover. If humidity is too low than they will hunker down. I agree that seeing pics of your vivarium would be helpful in determining if there is a problem or you jusyt happen to have a very shy frog!