Here is what happened:

You perhaps, put red oxide too "thick" so there has been little adhesion to the intonaco. Paints for buon fresco have to be very thin, somwhat of an "ink consistency" especialy at the beginning of the day. You need to apply several very thin coats to build the color. Also adding a little lime to the paint helps adhesion.

In your case I would suggest using "pompeian method" of simply adding red oxide to the plaster itself.