i think your only issue is the unibond make you are using..personaly i think its poor quality,,b and q do mapei adhesives now so i would advise you to stop using the unibond gear and change, personally i would fix 200 by 300 tiles with tubbed gear its about as big as i would go though,,many other tilers wont but everyones opinions are diferent.i think if you go back to b and q and get mapei tubbed adhesive or a decent
tile shop for a decent brand i.e mapei,bal,pci.weber it will solve your problem and buy some acrylic primer from the
tile shop also ,sounds like you are doing a fine job apart from that its just your adhesive letting you down id say.good luck