Is the sub your using a moss? Moss is notorious for causing impaction when ingested because it just absorbs more water and keeps getting bigger. I use Eco Earth which is coconut husk ground up into a fine soil. It is hard to see poo on the sub because its the same color but I just make it a habit that every day or every other day I take everything but the plant out and do a poop check. Sometimes I have to move my hand around in the soil to find it and sometimes I dont find it because she often poops in the water dish.

In my opinion its the best sub because even though she ingests some of it, its so fine and dosn't swell when exposed to water so the small amount she ingests it passes easily threw her system. It dosn't keep humidity as well as moss, though, but large leaf live plants keep the humidity pretty well as far as I can tell.