I would not worry too much. I have had some lay at 5 months and others at 11 months. Make sure they have a nice nest and put a golf ball in it. Sometimes they sit in there and practice.
First question is - are you certain that these birds are, in fact, pullets? If not, your wait for eggs could be a whole lot longer.
At six months of age, there is no reason for concern that they have not yet begun to lay - as long as you are providing a good environment and proper diet they will lay in their own time.