Mob spawning on servers works a bit different than in singleplayer which can have unexpected results, especially when trying to use the spawn mechanics to build a mob farm. Those mechanics can be quite complicated, you can find more detailed information about them here: https://minecraft.fandom.com/wiki/Spawn
This article only covers unexpectedly low mob spawning which is a common problem on servers when building farms.
The amount of mobs in a dimension is limited. This limit increases depending on the loaded chunks, so more players in different areas raise that limit. However, a player in regular chunks loads in many more mobs than a player that intentionally only loads few spawning platforms in a specialised farm. This results in low spawn rates in mob farms while other players are in the same dimension.
In Vanilla, there is no easy solution to this. You can only try to coordinate with your friends to only use mob farms while you are alone in a dimension (e.g. use your Overworld Creeper farm while your friends are exploring the Nether).
You can only use the methods mentioned below if you install the mentioned software!
On Spigot (Install here) and Paper (Install here) you can increase the base values for the mob spawn limits in the bukkit.yml file. See the documentation here: https://bukkit.fandom.com/wiki/Bukkit.yml#spawn-limits.
On Paper (Install here) you can also enable the per-player-mob-spawns option in the paper.yml file which replaces the global limit with a limit per player. This should help with running farms. You can find the documentation here: https://paper.readthedocs.io/en/latest/server/configuration.html#per-player-mob-spawns
Please note that increased spawn limits can cause performance problems, so be careful when changing those options.