Check out the Chance of the randomspawn: if it's too low, that might be the issue, so bring it to 1 in order to check if that was the issue. If it was not, continue with the steps below without changing it back until the issue is solved.
Check the world difficulty: if you are trying to spawn an hostile mob type, it shouldn't be peaceful!
Remove any limitations, such as biome and conditions, then see if it works
Check if there is any mispelled word anywhere, such as:
the mob internal name
the world name
If the RandomSpawn starts to work after deleting all conditions, start to add them back one by one, and see which one(s) caused the issue in the first place. If you are unable to understand why the conditions thus found are problematic, ask for help over our discord
If the randomspawn is still not working, continue debugging by taking the actions listed below, based on the randomspawn's Action
ADD Action
Make sure the plugin's spawning configurations are correctly set
(Below MM 5.6.1): the config file is located at: /MythicMobs/config.yml
(MM 5.6.1 or above) the config file is located at: /MythicMobs/config/config-spawning.yml
Make sure you have enabled GeneraSpawnPoints in the config file, or else the ADD action will not work
Make sure you are not in survival or adventure mode, as other gamemodes will not work
If all of the above fails, ask for help over our discord
REPLACE Action
Check the world difficulty: regardless of what mob type you are spawning, it shouldn't be peaceful! This action can only replace hostile entities!
Make sure the world can spawn monsters naturally (For instace, by making sure that the doMobSpawning gamerule is set to true and other such things).
You should know that, since the Replace action will only replace hostile mobs, you must make sure that those mobs can spawn first, according to theirs vanilla Natural generation conditions.
Amy