Sounds like the installer should watch Ken's DriTac video. Perhaps the installer didn't roll the floor, but then some adhesive manufacturers do not reccommend that. Not sure why but the hollows are often because of not rolling or weighting the floor down when the glue is setting or clear lack of using the wrong trowel.
Even then if one does this rolling or weighting even old Pro's like Mickey Moore, NOFMA's Technical Director wrote an article saying some hollows are just inevitable and do not affect the performance of the flooring. But can be annoying. https://www.youtube.com/watch?v=RwBoU2i ... z2KB0.dpuf