The same thing happened to me and I could't figure out why. I know why and it's because the creator only let this work for a certain gun mod (he says in the description) so it only works on the gun mods he did, so you need to download those but the default guns work as well. Another thing might be that you have a blood mod that disables it, that was my case, I have a bunch of blood mods enabled and I disabled them all and saw that about 2 of my blood mods made the stains not show, so you might wanna check too see if any of your blood mods are going against this mod.