Slimes do not spawn in SMP [closed]
SSP normally has a higher mob spawn radius around the player. This means that there are more places for other mobs to spawn first. Because slimes spawn less than other mobs it may reach the mob cap before a slime can spawn.
You can test this by changing view-distance=10 to a lower number in the server.properties file as this will give mobs less spaces to spawn outside of your farm.