It seems Duffy negativity does not offer a 100% resistance to vivax malaria in Africans; any clues as to why this is so?????

Because despite of Duffy antigen negativity in some cases, they catch the infection as others. This suggest that the parasite change its strategy to invade the RBCs through attachment to these receptors.

For many years it was believed that Duffy antigen negativity gave complete protection against p.vivax.
However, in studies conducted mainly in east Africa have detected p.vivax in Duffy negative individuals indicating that the parasite may be evolving alternative invasive pathways