A common cause is that radical formation initiated by atmospheric oxygen is inhibited by the fatty acid radicals. lt could also be possible that, although driers have been used, their activity appears reduced as a result of adsorption by the pigment.
The brown pigment you mention should not really cause these problems although the possibility that it might yet do so cannot be totally excluded.
In this case, the type of alkyd resin used could conceivably play a part – as could a synergistic effect of other components. We ould suggest a radical change of formulation.