another problem may be in the 70's/80's tile manufacturers made self-spacing tiles, (tiles whitch had small lugs on them so you could tile with out spacers) good idea at the time but as walls moved they fall off after a while, i'm seeing it alot at the moment! if your tiles have a very small...
i go along with jason on this one, BAL make a product called 'mosaic fix'
which is brilliant and is brilliant white, to match their 'microflex' or 'superflex grouts' so if the adhesive comes through you can't see it.