Setting cap stones on a stone veneer wall

I am building a thin set stone veneer wall for my back patio. The place where I bought the stones said I could use the same mortar mix for the cap stones as well the veneer. The mortar mix is 4 parts sand, 1 part type S mortar and 1 part portland cement. I am now finding the capstones are loose. The capstones are made of Pennsylvania blue stone. Should I use just a straight type S mortar to set the stones?


I should mention the wall underneath is cinder block. The sand/mortar mix sticks very well to the cinder block, I just spent the past hour chiseling off the mortar from the top of the wall where the cap stones come off. Should I pretreat the PA bluestone with anything like muriatic acid to clean it?

 

When you install the caps are you pressing the bluestone down into the motar bed. (I use a rubber mallet) or are you just laying the stone on top. The joint should be compressed.
You could also brush the bottom of the stones with portland cement and water before you set them.