This is known as 'silvering', and is the effect of the clear carrier film being applied onto a matt surface. This causes air bubbles to be trapped beneath the decal, and the clear areas are also acting as a 'seal' over the matt paint.
The solution is to apply the decals to a gloss surface, preferably with the aid of a decal softener and or setting solution, such as Mico Sol and Micro Set, or similar solutions.
A gloss finish can be achieved by either using gloss paints, such as the huge range of camouflage colours made by Xtracolor (enamels) or Xtracrylic (acrylic), or by applying a gloss clear coat before the decals are applied. Many modellers use Johnson's 'Future' for this, which is a relatively inexpensive, acrylic floor polish, although since the change of formula a couple of years ago, results have varied sometimes.
Alternatively, a good quality clear gloss varnish can be used.
Once this has fully set, and the required smooth, gloss finish has been achieved, the decals can then be applied. When fully set (leave them at least 24 hours), given that they have settled down correctly, then any decal adhesive residue, or light staining from any decal solutions, can be removed by gently cleaning with a damp cloth. When totally dry, a further gloss coat is then applied, in order to seal the decals and obtain a uniform finish.
When this coat has set, then the required final finish can be applied - that is, gloss, matt, semi- matt etc, by applying the relevant clear varnish.
Have a look at some of the build threads here on the forum, and you'll see this process in action, with a variety of materials/brands in use.