I have had this happen before. It was off an old sheet. I am not sure if it is from them being old or if the decals were printed on a thinner stock. Sometimes, I will go through several decals before I get one to set without breaking apart. I have since ordered a new sheet.
Not sure how you are applying your decals, but; I have started using this process. I cut out my decal, trimming as close to the decal. Place the decal on my finger, face up and apply water with a brush to moisten the decal. When the decal is loose, I use an x-acto knife and pick up the decal still on the paper. Put it in the vicinity you want the decal located and using the brush, slide it off the paper. Use decal set to float the decal and position it. You won't need a lot of decal set, a little will allow the decal to move around. After positioning the decal, use the brush to remove the excess moisture. Be careful to not move the decal. If you move it, it can be repositioned while you are removing the moisture. I hope this helps.