Kleinhanz, you may be on to something. I haven't seen this yet, infact, other species of mantellas calling usually triggers different ones to call. I don't know in dendrobates though, it is a possibility.
Lydia, your tanks look great. Perhaps certain species of plants would benefit them more. Find out what the original keepers plants were, and maybe pick some up, and either replant it totally, until they like it, or, add to it? The reason I am thinking this is because I have 5 mantella laevigata's who would not call really, or anything for almost a year. I had them in a not so thick planted vivarium with bark, orchids, and just general tropicals. I then redid them a 29 gallon with waterfalls, and various ferns found in their original habitat, and they just went bonkers. Within 6 days I had 9 eggs. It is now Day 15 and I have recieved over 20 eggs from them. So, just keep trying until you find something they like, and if they can see through to the other tanks, perhaps put up a blocker, like an aquarium background so they cant see other frogs. That might make them shy if they can see the other species. ANyway, good luck with them, I know how frustrating shy frogs can be sometimes, especially when they previously were very bold.
Ed Parker