Pinholes are normally caused by not forcing the grout into the tile gap. A thin skin of grout can bridge the gap, but when it dries the pinhole is exposed. Could also be drying cracks that are first showing up as pinholes, they're normally caused by too much water in the grout. As the waer leaves the grut in teh curing process, microscopic voids are left behind. These eventually join up to cause visible pinholes/cracks.