I set up a skeleton mob spawner with all usual thing for that version and tested it in a nother world and same thing happend where the mob spawner does`t spawn fast and i lit up the caves close to the spawner because i thougt that was the issue but it didn`t fix the problem.
In a previous version i did the same thing and the spawner spawned mobs normaly.
I folowed this design blelow and still works
Comments 2
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I might have a solution to what is happening, the monster spawners have a larger dedection range than you would think (9 blocks from centre), and if too many skeletons are too close it the spawner it won't spawn any more or have a very slow spawn rate (i learned this the hard way making a zombie/drowned farm myself). So in that regard i think everything is working as intended... all you would have to do is extend the path before the skeletons fall after the bubble elevator so that they are out of range after they drop so that the spawner just keeps pumping them out. Of course this is only the case if you are letting them build up before you kill them, if you are killing as they drop its less of a problem and spawners can be naturaly be slow as well...
I hope this can help you and your troubles.