Try leaving the egg there overnight.....
I went to the Discord mod support channel last night to ask about this. Pathoschild confirmed that he was going to check if this was happening in the vanilla game too. For me, this started happening after the 1.6.3 update on Steam, so it might well be an actual game bug and nothing to do with mods - after I had dissed a ton of mods trying to fix it myself
Last night, I left the green egg floating from the incubator and went about my day - then slept and woke up to the message that 'A male slime has hatched' - and there he was! So now slime egg #2 has been put in... the same egg is waiting, Look Up Anything says I will have a new egg in xx hours, or whatever - we'll see if I get another slime in the morning - fingers crossed!
If Pathoschild finds that this IS happening in the vanilla game, we'll have another patch from CA in due course :)