  (Abridged) Globular clusters host multiple stellar populations showing
different sodium enrichments. These various populations can be observed along
the main sequence, red giant and horizontal branch phases. Recently it was
shown, however, that at least in the globular cluster NGC 6752, no sodium-rich
stars are observed along the early asymptotic giant branch, posing an apparent
problem for stellar evolution. We present an explanation for this lack of
sodium-rich stars in this region of the colour-magnitude diagram.
