I'm inclined to agree to a great extent with the
others. You seem to be saying that the $900 job
took less than one day. That's pretty steep. And
why are they squirting in adhesive? What do you
mean by "adhesive"? Hopefully this wasn't tile mastic
on a concrete floor. And why are you walking on it
the next day? There seem to be details missing.
They should have stuck down the loose tiles with
thinset and then grouted the next day. Thinset is
generally a 2-day cure. It shouldn't have been
walked on the next day except to grout, and then
only carefully, with something like a sheet of plywood
to spread the load.

My best guess is that your whole floor is probably
going and is likely to leave you with two choices:
Redo the whole thing, hopefully breaking out the
old tiles first, if you can, or get your self some
thinset and grout, then just re-stick tiles as they
come loose. The latter solution will mean, of
course, that you'll also have to accept putting in
some new tiles that probably won't match.