Ok sounds like u used a satin gloss or impregnator so you need to take it off with HG Remover and then when its dry re-seal it with Lithofin slate seal.
Try this 1st might save your a$$...........get a ltr of slate seal and put some on a couple of tiles leave it for an hour see how they come up it may work if you have used an impregnator.
If they don't u will have to strip them as above and it will be tuesday before you can reseal i would think
gents please accept my thanks, I will know exactly what sealer was used tomorrow, can you tell me how i can find you tomorrow if it is not the one stated
cheers