Floor paints are, in my humble opinion, at best, problematic. There are so many reasons why a floor paint can fail. Wet cement from rain being tracked in by feet and tires; high alkalinity; poor quality paint; and on and on.
First thing to do is to remove what is loose. If your garage is built to withstand such a thing, use a High Power Pressure washer (at least 3500 PSI) and be really agressive with it. You may also use a Sodium Hydroxide paint remover with this process to get back to as much of the origional surface as possible.
Allow this to dry for a day and then thoroughly with the cement with White Vinegar and rinse it thoroghly with the power washer.
Allow this to dry, without any foot or tire traffic for several days, prefferably a week. Then repaint with an ALKYD