Are you perhaps soaking the decals too long and losing the glue? One of the most common problems is leaving the decal in water for too long.
Suggestion: You don't want to leave it soaking in a container of water. Dip the decal in water, but remove from the water and set aside on a hard surface and after about 30 seconds, check to see if the decal slides freely on the backing paper. If it doesn't move, leave for another 10 seconds and try again.
It may simply be that the decals are crap. I've had some Airfix/Heller decals which simply refuse to stick and are inert to some decal setting solutions. One set on a Heller kit from the mid-80's was moderately thick, brittle and lacked adhesion. Another set from the early 00's was extremely thick , but reasonable flexible and also lacked adhesion.
Not all decal setting solutions are the same. I had one occasion where the decals on a Heller model were unresponsive to MicroSol, partly responsive to MicroSet and went crazy wrinkly with Gunze Mr Mark Softer.
But I digress.. As I mentioned above, as a first step, try to avoid over-soaking your decals.