Probably mob capped is filled, which is usually what happens when you have multiple players on the server. Maybe light up the area more to increase the chances of spawning.
I suggest using Amidst or chunkbase (I prefer Amidst) for more accurate .
Since you guys digged already,why not dig the whole slime chunk? So there won't be any obstruction whatsoever. I suggest you watch gnembons video about slime farm mechanics and their quirks when it comes to spawning.
Thanks a lot! We will try to dig the whole chunk hope it works then