It's strange that only full tiles are affected (according to photo).
Did you use a wet cutter for the bottom row?
These tiles do absorb a lot of water. If they are sealed whilst they still contain water then the water will be trapped within the tile.
When they are being sealed you will probably hear a 'cracking noise'. This is the glaze on the tile still crackling. I would seal again and again.
As you have now found out, these tiles are not really suited to a shower environment. I personally would not guarantee my work if I installed in a shower. Some customers are very insistent, like the ones that have slippery polished porcelain in their bathrooms!
As an experiment measure out some water and pour it over a tile in a dish. See how much water the tile absorbs.
How long did you leave the tiles before you sealed?
I would suggest you also contact Walls & Floors, but I have dealt with them in the past and they are not experts in tiles or anything tile related.
