So far, only flies are spawning around the world? Any fix on this

im also having this issue! have only seen flies in the plains and forest biomes, and terrapins and lobsters in the water. I installed the mod for an existing world but I assume there shouldn't be an issue with that normally.

Also having this issue as well especially eregious when other mods are added that effect mob spawns

I'm having the same issue. It seems, strangely, that Alex Mobs only spawn in non-explored areas. If you've already been there and installed Alex Mobs after it will not spawn any of AM mobs there. You need to go to new areas.

Ironically I have the opposite effect, the terrapins keep appearing even at 0 spawn weight. I had to use another mod to keep it from spawning.