I've laid polished limestone tiles in my kitchen and am using Proflex Ultra grout. However, as soon as I apply the grout, the excess starts to 'crumble' immediatelyon contact with the other tlies. This seems to be the water being drawn out of it but I even wiped the edges with a damp cloth prior to grouting but it's still drying too fast. Can I do anything about it as this bag of grout is not going to last long!