Hi @Linda_Abenaim,
To add on to @gregcaporaso's excellent advice, I think its worth considering a couple possibilties.
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There aren't differences at phylum level. This wouldn't suprise me. Members of hte same phyla can have very little in common and still be grouped together. For example, humans, sharks, and lancet worms are all part of the same phylum (phylum chordata) but beyond having spinal cords and bilateral symmetry, there isn't much we share from an ecological perspective! Pat Schloss did some interesting work a while ago which suggested family was a sweet spot for colorectal cancer research. I dont know enough about insect guts to know if this will hold true or if you'll need to go deeper, but family tends to be the limit of my taxonomic levels.
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Statistical power due to dimensionality. If you're seeing a difference in beta diveristy but not detecting differentailly abundant taxa, it may simply be an issue of statistical power. ANCOM is conservative and often has issues with small sample sizes. This can be managed by filtering your data. The good news is that ANCOM-BC which Greg recommended does osme filtering for you, so you should have fewer concerns about low abundance/low prevelance features decreasing your power.
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Low statistical power due to sample size. Sometimes, a sample size that is big enough for a difference in beta diveristy isn't big enough for a difference in differential abundance, or you're underpowered to detect certain samples. I dont know about your system or your expected effect size, so I'm not sure where you fall. In my system of choice, I worry at fewer than 50 samples, but I also deal with free living organisms who have a tendency to do what they want.
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There is no signal. It's possible there just isn't a difference between your groups, or there may not be a difference you can detect between your groups. This might be indicated by a lack of signal in beta diversity, especially when you test with something like adonis. Unfortunately, there's a publication bias toward positive results, but it doesnt mean your negative results dont add information.
Best,
Justine