I have the exact same problem with white latex paint on my garage floor... I was super skeptical about citristrip. It had a lot of bad reviews. I'm guessing nobody that tried the stuff let it sit for a full hour like you did. You have made a full believer out of me. Thank you.
i painted the wall and ceiling of garage with latex paint without drop cloths. The spots n the the bare concrete floor are extremely difficult to remove with chemical, wire brushing with drill etc. Small spots ground into the pores of the concrete that are impossible to remove. I intend to paint the floor with 2 part epoxy paint. So I guess ground in paint may not be an issue ?