Mobs becoming invisible in Minecraft?

So I've recently started up a CraftBukkit server in Minecraft and an issue that has recently occurred, is that mobs, passive, neutral and hostile, are now all invisible. I know for a fact that they are spawning because I summoned a Cow just to see if they simply were not spawning. What I found out was that when I executed /summon Cow, nothing spawned, or so I thought, after a few seconds I heard it "moo". I was baffled at first so I naturally looked around to see if there were cows in the vicinity, but nothing. I tried again but only with a spawn egg and I had the same results. So I then restart my server and tried again and STILL got the same results as before. I did some research and came to the conclusion that it could be the actual CraftBukkit download itself, but I'm skeptical, only because this has never happened before with the same CraftBukkit build.

From what I've gathered, the likely result could be that the problem is within my "worlds" folder because I've had to replace that folder before due to terrain generating issues. The mobs have always been able to be visible before in the previous folders as well. I also have check my server.properties file just to make sure that my spawn-animals and spawn-mobs are both set to true and still have no visible mobs, only sounds.

Solution 1:

It might be because you have to download both the resource and behavior pack. cause if you only download the behavior pack, the mod will be there, you just won't be able to see it because the resource pack creates the image of the mobs in the mod, and that's why you have to include it sometimes and without it, the mod won't be visible on your screen, hope this helps.

Solution 2:

Make a repeating command block that runs this command:

effect clear @e invisibilty

See if that works, someone might've made another command block that gives entities invisibility.