This issue was brought to my attention by Mingo39_. I have not heard of it happening to anyone else, so if this has happened to you as well, please reply to this thread. As you can see in the video, flames around the mob spawners indicate that more mobs should spawn; however, you do not see more mobs spawn nor does the slime stack count increase above the existing slimes. The slime spawners are in a slime chunk and this issue happens with other mobs, both passive and hostile as well; however, it's sort of an on/off thing.
Slysu reported it too shortly after you left today. It was happening at a skeleton spawner, which started working when I went to investigate and then stopped working after spawning just a few skeletons. Other spawners (including my own skeleton one) were working fine ata the time.
The plugin has difficulty keeping up with the slimes as they are constantly changing when they get killed.
I noticed with my skeleton grinder that it would allow about 5 skeletons to spawn then it would stop. when I killed these skeletons it would allow a few more to spawn. I don't know if this was a coincidence or intentional
I also would like to see this issue fixed quickly! I'm having trouble with skeleton and sheep spawners on my own island. And noticed it on 2 others.
