I have a slate walkway that is a concrete base with slate tiles laid on top. The tiles are about 10 ins by 20 ins and 1/4 to half inch thick. There's a section of about four tiles that comes loose every year. I've had it fixed several times, but always the same group of tiles comes loose. We hardly ever get a frost. I suspect it may be related to those tiles getting very hot in the summer with the sun shining on them for a few hours, the tiles expanding and coming loose from the tiling mortar or cement that is used to stick the tiles to the concrete. What would be the best way for me to fix them?