Hmm. The 'milky substance' is the decal adhesive mixing with the carrier film residue. When you apply the next ones, don't use 'Set', but put a lot of water on the area to be decalled, using a paint brush. When the decal is positioned, use the same brush to wash the residue from under the decal, then press it into place. See if they'll settle without using 'Sol' first, but maybe put a drop or two of 'Sol' on the decal whilst still on the backing paper, after removing from the water.
Let them dry on the model, then gently clean any marks/residue, again with a wet brush, patting dry with a soft cloth, NOT a tissue. The latter will leave 'dust' on any sticky residue, agravating the problem.
Once they have set, then, if there is any crinkling etc, apply a little 'Sol' and leave them, unless there are obvious air pockets underneath. If so, prick with a pin or point of a scalpel blade, press gently with a paint brush, then 'feed' a little 'Sol' into the hole with the tip of a brush.
Sorry mate, can't think of any other 'fixes' at the moment, not without physically seeing the problem.