Tile adhesive on tiles

[wyse85]

Hi,

Laid some wall tiles 600*300 unglazed porcelain in bathroom using mapei keraflex and have noticed that there are certain areas of the tiles that i cannot seem to remove the adhesive from. i can wash them with clean water and they disappear but then it comes back through when dry. The tiles were sealed with Lithofin fz prior to fixing. is there a particular cleaner that would help me out with this? or a home made cleaner that would do the job?

Thanks