bristlenose need deep, narrow caves that offer exclusive access, with a non threatening doorway. frontosa require an open floor area approx. 1 ft. x 1 ft., with another 3-4 ft. area that can be moderated, and cleared for safety, by the male involved. your bn's will pair up themselves when ready. fronts same, but too many males, will reduce the the probability of any one male clearing enough space to comfort the female into the spawn. for that reason, only one of your 5 should be a male. to keep more males than that, in a breeding group, would require the addition of many more females. HTH.
edit to add: over time, if no breeding activity is taking place, the 'failure' is most likely caused by the male's inability to create a non threatening area for a safe spawn. there are many reasons why this could happen within the confines of an aquarium. incompatibility of species is likely in the top three.