You need to seal the decals after they are on your model. But your problem might just be how you handle the decals... As a matter of fact, you should handle them as little as possible, because once wet, the glue that's under the decal will easily be removed. Always push the decal off the backing paper onto the model (I use brushes, toothpicks,.. to do that).
For those models that have decals peeling off, try to re-glue them with a mixture of wood glue and water. Let dry ten seal.