Mob Spawn Rate Messed Up?

My mob farm was working just fine RIGHT before I updated. Then I updated my server about 45 minutes ago because my friend couldn’t join unless I updated. When I came back, my mob farm has been empty ever since. I have been standing in my usual AFK spot for 20 minutes and have only gotten 4 zombies, as opposed fo much MUCH more mobs. I just died yesterday and I’m trying to get my 52 levels that I lost, back, but now it seems as though that will be impossible.