Certified GKH Gardening Expert
After a time, lavender tends to get very woody at its base. It is not as likely to have new growth on very woody stems. The stems closer to the top are not as woody, so they will produce new growth better.
You may want to consider either replacing the whole plant or starting a regimen of rejuvenation pruning where you remove the oldest stem or two each year to encourage new younger stems to grow from the base.