How to get minecraft xp farm to spawn mobs? [duplicate]

There are many different things that could be causing this:

1. Mobs will not spawn within 24 blocks of you. Ensure that you are waiting outside of this radius from the farm. If it's a 23 block drop then you should be fine so long as you're not directly under it. Also, being too far away from it will first prevent mobs from walking and falling into the drop, or prevent them from spawning at all if you are far enough away (around 120 blocks I believe).

2. They will not spawn on peaceful difficulty. One of the more obvious ones, but a potential cause. Turn your difficulty to easy, normal or hard.

3. To spawn, zombies, skeletons, witches and creepers need two blocks of vertical space and an opaque block under their feet. Spiders require a 3x3x2 space, and endermen a 1*1*3. Ensure that your spawning pads meet these conditions (not a half slabbed floor/roof) for the mobs that you want to spawn.

4. Mobs will avoid falling more than 4 blocks. If it's more than that then you'll need something to push them in (piston, water, other mobs). I don't think the sign trick works as of the 1.8 snapshots. Some might be spawning, but then just despawning as you wait and not falling down.

5. If you have messed around with /gamerule, make sure that doMobSpawning is true. This gamerule, if false, will prevent any mobs from spawning.

6. If it's none of those, it may just be that your farm is too small. If you have not lit up caves then the rates will be very slow, especially at night. You can fix this by expanding your farm, or lighting up nearby caves. As you're on creative you may wish to start a superflat world (where there are no caves) to get better rates.