Can anyone help me please. I have started laying a patio using indian sandstone (22mm calibrated thickness purchased from Travis Perkins). Am bedding down on a sharp sand (not that sharp to be honest) and cement mix(6:1) at about 50mm thick over a compacted 20mm to dust sub base. My problem is that quite a few of the slabs are not bonding to this mortar mix and are just lifting. The mortar below has hardened perfectly, its just the slab is not bonding with it. Would it be a good idea to pre paste the back of each slab before laying with a neat cement / water mix to aid with the bonding process or is there a better way. Never had this problem with manufactured slabs before now. Is it something to watch out for with sandstone. Any advice please.