Trying to attach decorative stones to the house foundation, but they won't stay in place. The decorative stones are made of regular concrete, approximately the size of a brick and about 4-5 cm thick. I've tried different mixes ranging from 2:1 to 3:1 with sand and cement, with consistencies from thick porridge to thin soup, without good results. I prepared the wall with a grooved finish and waited a day, applied directly and also tried moistening a little before. I've also tried applying about 0.5 cm up to 2.5 cm of mortar on the stone and made an indentation in the mortar on the stone before pressing it against the wall and twisting it a few cm clockwise and counterclockwise to try to create a vacuum. Pressed firmly with quite a bit of force and held it for a little while before letting go, but no luck. I'm starting to believe the stones might be too heavy.