are the leaves that have yellow tips, dark green or light green? like bejay said, a picture is the only thing that will allow us to help you out totally.

but, if your leaves are dark green- the yellow tips are probably a sign of a nute burn. if your leaves are light green- they could be deficient.

my guess is a burn. when MJ plants start yellowing because of natural N deficiency in flowering, they don't start at the tips. the whole leaves just sort of pale.