Similar boat. From what I've read, only about 50% of the final result is achieved (at best) by the 6 month mark. When you consider that real growth is not supposed to really kick in until month 4, it would be difficult to see the finished product just 2 or 3 months later. I asked Spex on another thread roughly what % of hairs should have penetrated the surface by month 6, and I believe he said roughly 40-50%.
I'm approaching 6 months and have a similar situation. Some areas the hair is thick, even matured. Other areas I see little hairs that have sprouted but simply need to grow and mature. And there's others, like you describe, where gaps exist and hair hasn't penetrated yet. I have a magnifying mirror, and at close inspection I can see some really really fine black dots waiting to penetrate. But still, others where nothing is showing yet.
I've been told time and time again that 6 to 9 months is the sweet spot. There's a lot of variability. Someone may grow a lot early then level out. Others may grow at a painstakingly slow but consistent rate. I would talk to your clinic, as I have. Trust me, I thought I'd be farther along at 6 months, but progress is there. Try and keep perspective. You're still early at 7 months man.