With some mods, most of the mobs on the mod itself aren't appearing. This needs to be looked into. Some of the only mobs I've seen are: The Rhino the Dropbear and nothing else
My mods that might cause the problem are biomes o plenty and domestication innovation although the last one seems unlikely. Ive also got the emerald lasso mod, I don't think this causes the problems because I've got it installed and the two mobs I've mentioned before still spawned.
Still ill try to look into the mods I have downloaded and will report if any of the mods are preventing some of Alex's mobs from spawning.
Even on vanilla biomes they still don't spawn.

i've had a similar issue happen in the past with other mob adding mods, it COULD potentially be due to the spawn rates of other existing mobs added by other mods your using that are just essentially eating up the "space" that mobs from alex's mobs would spawn into when the area isn't already bogged down with to many entities. hence the weight settings in the configuration files. on that note, check to see how high the numbers are set in that on your end as high numbers means lower spawn chances to begin with. it works via ratio - so 1 in 1 chance is always going to spawn while a 1 in 20 has a 1 in 20 chance of being able to spawn.

Although this issue you describe could legitimately be a bug that's causing certain mobs from the mod not to spawn at all regardless of their config settings, try out the above suggestions first and see if they start spawning in. if they spawn in when their spawn weight is set to 1, then that means the mod is working find and its something else in your mod pack that's been spawning into those biomes and taking up entity space tat the rarer alex's mobs would be using to spawn in with.

and while i understand its not a literal thing of space, I use the term loosely to give a better visual understanding to the reader as to what's going on behind the scenes for how Minecraft is choosing what it can and cant spawn in from the mod(s) in question and under what conditions they can or cant appear in.

I only know of these spawning issues/conditions due to past experiences running custom made mod packs that add alot extra mobs, such as from Lycanites mobs, ice and fire dragons, grimoire of gaia 3, ect all along side past versions of Alexs mob and have run into this dilemma myself back then. some things not spawning at all is just the risk ya run when ya got too many mob based mods all trying to spawn their mobs into the same biomes at the same time. in most cases some aren't going to have room to run, or in this case, spawn in at all'

Hopefully adjusting the spawn rates in the configuration files will fix the issue for you, but if not then come back here and give an update on the situation so the Alex can see that the issue is a persisting one.