It's a bit of a misconception that adding a handful of isopods and spingtails to an enclosure means you no longer have to clean it or concern yourself with managing the waste from a relatively large animal like a blue tongued skink.
It seems this misunderstanding largely comes from the dartfrog hobby where it can hold true that properly constructed bioactive enclosures can run for long periods of time without any 'cleaning ' but this is because of how tiny the actual dart frogs in question are and the high humidity environment they require. To achieve the same effect for a blue tongue skink you would need to be keeping it in a truly huge enclosure to accommodate enough custodians to enirely break down it's waste.
To put it in perspective I keep tiny geckos in bioactive enclosures that have thriving populations of 8 different species or springtail and 4 different species of isopod as well as millipedes, roaches, mites, rove beetles etc. and i can only JUST get away with not removing feces from the enclosures. But the geckos are so small that their vivariums are really oversized and provide plenty of refuges for the microfauna to reproduce. In order to achieve the same ratio of tank size to inhabitant size for a blue tongue skink I would need to keep it in a 20 foot long tank.
One problem you might also be having is that in a drier setup the springtails and isopods will tend to hide in buried or concealed areas with a more humid microclimate whereas your skink will presumably defecate wherever it likes but frankly you are unlikely to achieve high enough loads of microfauna in a BTS vivarium to completely process it's waste.
If anyone tells you otherwise they either don't know what they are talking about or are trying to sell you something 'bioactive'. Often springtails are feeding on the bacteria and fungi hat are actually breaking down the waste too and in a drier setup this breakdown might not be happening as quickly.
Your orange isos may be large enough that the skink has eaten most of them too, you might try incorporating tiny isopods like the dwarf purples that it's less likely to notice, as well as burrowing micro roaches like the 'little kenyan roach'. I'd also recommend the larger european springtails like Pogenognathellus longicornis or flavescens as these tend to do better in drier conditions in my experience.
Hope this is some help.
TLDR isopods and springtails will not be sufficient to process the waste of a larger animal like a blue tongue skink in any normal sized enclosure.
