Your crystals are floating? I've never seen that happen. If you can, give heptane a try for rex, it always seems to produce a more powdery product, and you can usually separate it to make a pile of orange, a pile of yellow, and a pile of white. This will let you test and see how pure you prefer your spice. They are all good, and all have a different characteristic or profile of experience. I'll use different fractions of the recrystallization depending on whether I want a more diversified experience, or what my mood is at the time.
I like to give my stuff 24-48hrs, but as I was telling someone else. Leave it long enough for the solution to be totally translucent, you don't want to be decanting naptha/heptane with any precip still in it. I've placed the same naptha that I had already let sit in the freezer for 48hrs, then decanted off the precip, back into the freezer for two more days and I'd guess I got another 10-20% more crystal growth and precipitation. Some things just take time.
Disclaimer: The "I" that has been referenced above is the character in a fictitious biography, narrated by BioTron. Any and all resemblance to actual events is unintentional, accidental, and/or coincidental in nature.