Hmm really interesting I never looked into decal and now it works fine at my end (Yey this will be useful for my explosions XD) I guess that the texture is the problem here for the decal, because I copied all what you have and its works fine anyway here a list of possible problems:
1.Bad texture you are using a bad texture get a new one (the market have infinity blade effects for free you can get one from there).
2.The material is bad or bugged, just create a new and try to change only what you need.
3.And last its a normal decal so if the terrain is not flat that can be the problem as well.
Good Luck