Hollow sound w flagstone dry bedding layer - What causes hollow sound?

[Conroe]

A 1000 sq ft flagstone (sandstone from India) was installed. Nearly every stone has a hollow sound (except the smallest stones 290mm x 290 mm) The bedding layer is a dry screed "engineered" aggregate of 2 inches. When 2 stones were lifted they did appear to be in good contact with the bedding layer showing the vertical ridges on the back of the flag. When a damp river sand was tried under one of the lifted stones, the stone no longer sounded hollow. Additionally, The resin jointing compound has failed and is crumbling and can be easily removed with a finger after rain. The installation contractor says he will fix the joints but that his installation method is correct. How can his installation method be right if it results in an extensive hollow sound issue? Why is there a hollow sound??

[digerjones]

The hollow sound is because the flag have not stuck to the bedding layer. I coat the back of flags with sbr mixed with cement.
Any pictures

[Conroe]

Yes I will send. Thanks for any help and insights. I don't see how to post photos to forum so will send separately to anyone interested in helping with this. Thanks!!

[seanandruby]

Go to top of main page, to noticeboard and photo instructions there.

[Conroe]

Photo shows the back of stone on left and the bedding layer on which the overturned stone was laying

Second photo shows the concrete base underneath the bedding layer and depth of bedding layer. 500 sqft of the patio has a concrete sub base. The other half has a sub base of gravel aggregate mix. Hollow sounding stones are in both areas. Same type of bedding layer across entire patio.

https://goo.gl/photos/8cX5cGXJbL8Dso5g9
https://goo.gl/photos/BfhaFetDD4YS2dWu7

[seanandruby]

Are you sure that when you tap it it is a hollow sound and not just a sound made from tapping? He should of been consistent with bedding to avoid differential settlement. Should be a wet mix for those type of slabs with sbr added and painted on the back to help adhesion.