Keep in mind that all limitations for vanilla slime farms still apply. Particularly, if you're not in a skyblock world, you will need to light up the caves within 128 blocks around your AFK spot to prevent other hostile mobs from taking up the mobcap. Also, since this is modded Minecraft, make sure no mob spawning prevention (megatorch and the like) affects the spawning area of the farm.
Can you verify the location of your farm again? The spawning platform should be chunk-aligned (F3+G to check), that chunk (not necessarily your AFK spot) must be a slime chunk, and when standing on the highest spawning floor, your feet should be no higher than Y=39. And, I guess, check that you are not in peaceful difficulty.