This is one that I've never had to deal with myself. But I've been asked this question an awful lot.
If my memory serves me correctly, in all cases where folks are struggling with that sort of thing, they haven't been mowing high, and their watering habits are not the best.
Can you describe what your mowing/watering/fertilizing habits have been like?
Without deviation from the norm, progress is not possible - Zappa. Tiny ad:
building a permaculture community with hands on gardening and natural building