Can Anyone Offer Advice On A Rookie Mistake?

[widler]

Bagged i find is easier to spread and use (standard set) tubbed is ok imo on a small splash backs ,

 

[Dan]

A lot of tubbed is good for kitchens wirh smallish ceramics. Always liked the white star by BAL myself. Never used any other brands. Think I might have used some Mapei of similar spec for a kitchen I randomly did a few years ago for a close friend thinking about it. They're still stuck solid.

 

[SJPurdy]

The walls were tiled before and they lasted for years before being removed. The tiles and addy layer (assumed) have been removed but any old primer that soaked into the plaster wall to reduce suction (if necessary) before the old tiling is probably still there. The ready mixed addys are designed to be used on plaster walls but if the wall is absorbent they need priming (to reduce suction) so that the moisture in the adhesive isn't sucked away to quickly leaving a (very) weak bond.
If all the tiles but the one that came loose are firmly fixed then I would assume that they will stay that way. The one tile that popped out may have done so for another reason eg a blob of contaminant in the addy.