Is there a trick to make red roof tiles and stone patio slabs stick to mortar?
We have a low wall which has a row of S-shaped red roof tiles set into mortar at 45 degrees to the horizontal along the top as coping stones. Some of these are loose: they are held in place only by the moulded peg on the tile, but lift off. I made up some mortar, laid it on top of the existing mortar and bedded the tiles into it. But the tiles have not adhered to the mortar: the new mortar has bonded to the existing mortar, but the tiles have not bonded to my new mortar. I can see the impression in the mortar of imperfections in the tiles, so there has been good contact over a large area.
Similarly, our patio has stone slabs (stone, as opposed to concrete) and a few at the edge of a shallow step are triangular parts of a full slab. I chiselled out the existing mortar and made up some more (I couldn't use the existing mortar because when I added fresh mortar as the "adhesive" it raised the slab too high above the others. I got the new tile nicely bedded in at the right height and level. But a week later one of the tiles hasn't stuck at all. The others are fine.
Is there something I'm doing wrong: should a roof tile or a stone slab bond to mortar that it is bedded into, or is it necessary to add something? I'm using standard ready-mixed mortar (*), mixed with water to make a moderately stiff paste. IN the case of the patio slabs I added a bed of mortar that was stiff enough that it can be moulded to an edge which will bear the weight of the slab without squeezing out, and then used a slightly more sloppy mortar as the top layer that the stone comes into contact with. Having laid the tile/slab and adjusted the position/level to match the surrounding ones, within the first few seconds of putting it in place, I have taken care not to move the tile again. The weather has been warm, and there has not been rain for several days after laying. And at this time of year the temperature has never dropped to freezing: I think the coldest temp was about 3 deg C overnight, several nights after laying the tiles/slabs.
When relaying slabs which have come unstuck. is there any alternative to using mortar, which doesn't require all the existing mortar to be removed to avoid the new mortar raising the level above the surrounding ones.
In the case of the stone slab, I didn't get as far as re-pointing the gap between other slabs alongside it: I suppose if I had that pointing, it might provide additional adhesion and support to hold the slab in place.
(*) It was described as being suitable for bricklaying and laying of patio slabs. I forget the proportions but I seem to remember it was about 80% sand
20% Portland cement, so a 4:1 mixture which seems to be about what is recommended.