Hollow sound to porcelain paving.

[PeterDH]

This is my first post so I hope I do this correctly.

I've have had approximately 200 porcelain tiles laid outside on a wet bed of mortar and an adhesive slurry was used on each tile. However, I'm finding that many tiles sound hollow in one or more corners of each tile. Paving expert says the minimum contact/bond area should be 70% minimum and I think some tiles may have less contacts than that. All the tiles have been grouted with a suitable exterior flexible tile grout and there is no sign of any movement.
Should I be worried about the hollow sounds which obviously mean that there are gaps underneath and no bond in those areas ?

I welcome any comments and advise about this.

[SureSet]

Could the mortar be coming off from the supporting layer?