Too many folks use cement instead of tile adhesive...tile adhesive is much more expensive but well worth the additional cost. Using cement will normally end up having tiles pop up, cracked grout and needing replacement after the floor has started taking weight.
For hollow spots, that is normally because the concrete slab wasn't properly leveled and the installer didn't explain this to the home owner or decided to hide it.
We lost touch with the folks that did the tile in our house so no help there. Can't say they would have been great on their own...I was right there to explain how things needed to be done.
Agreed - just mix it and pour it yourself, as long as the substrate floor is clean and grease free - also, as it goes on as a liquid, it's self levelling.