They start of sat nicely on the wall - but when the adhesive goes off
it appears to lift away from the wall - leaving the tile sounding if
not fully - at least in part hollow when you tap it - meaning that it
isnt' stuck.

I remove them at that point - clean off adhesive and try again - but
it only seems to be in the very final stage of the adhesive going off
that this happens - as the moisture from the adhesive has definately
contacted the wall as the plaster on the wall is left damp.
If you're mixing the adhesive from a powder, I'd say it needs to be
sloppier. All these things hang on initially by suction, replaced by
adhesion as they dry. Try slapping a load of ready mix tile adhesive on the
back of the tile and pushing it flush with the rest. Gives you a lot of
excess to clear off but that has to stick.
Bathroom floor I did last week just won't dry in this weather. Solid floor,
shady location, unhappy customer.......