Was it allowed to get damp before painting?
Google suggests this is a characteristic of idigbo...
http://www.woodconservatory.co.uk/whyoak.htm"...most idigbo conservatories are painted due to the wood staining that can occur".
http://www.eoburton.com/Timbers/Idigbo.htm"The wood contains a dye which may stain other materials when wet; it also contains tannin which causes staining when in contact with iron under moist conditions."
You anywhere near a dusty iron works?