I do wonder whether the slowdown in breeding is related to the fact that many of the animals in Australia (particularly those originating from Melbourne) are subspecific hybrids. The population thread notes that the females imported to Adelaide in 2020 are purebreds, and the recent annual report from Zoos SA indicates they are being paired with the males for breeding (unsure if the males are purebreds or hybrids).
Were colobus ever displayed at Monarto, or were they housed off-display? Colobus were included in the Monarto masterplan in 2015, although I'm not sure whether Zoos SA plans to house them at both sites going forward, or whether the move to Adelaide (as an African rainforest species potentially to be alongside gorillas in the future) is simply a swap between the two campuses.
(Also, more than happy to continue this discussion in another thread if it's more on-topic elsewhere.)