In one of my greenhouses I have 2 M. duwei plants, standing right beside each other. Both were grown from habitat collected seed. One of them produces hooked centrals, the other one does not have any centrals. In a more sunny position in another greenhouse, I have an old seed-grown plant of M. duwei, without any centrals. In nature, about 10% of the plants form hooked centrals (but not on every tubercle).
I have the impression that the sunnier position your M. duwei is in, the smaller the chance is that hooked centrals are produced. The FitzMaurice greenhouse, which I visited a few times, is quite shaded, and therefore the plants of the Fitz Maurices do not receive very much direct sunlight. The plants of the Fitz Maurices are quite elongated, much more so than you see in European greenhouses.
M. magallanii (and several lasiacantha related taxa in the Viesca region) form hooked centrals when they are very young, but when they get older the formation of central spines is an exception.
Wolter ten Hoeve, The Netherlands.