I saw the pictures on your site. It looks like the spuitplamuur is bubbling due to a greasy underground. Stabalit Express is a little greasy when applied (even when mixed perfectly). I usually add an extra thick layer of it and then sand it down. After that it is less greasy and to be safe you can paint the little spots with "kunststofprimer" (for sale at Gamma and Karwei), and then use "spuitplamuur".
Danny v. Dijk
PS: sorry englisch viewers, some things are very hard to translate for us dutchies.