Re sealing a concrete block walled fountain.
If your blocks are like those in the UK they are porous, your attempt at sealing them may have made an subsequent attempt at grouting, plastering or rendering them unlikely to succeed as there may not be much for the 'plaster' to get a grip on.
If that is the case then lining the fountain with a normal liner may be the bet solution, or simply rebuild the wall and 'plaster' the walls before repainting.