Arrgggh I wrote a long reply earlier and then DD3 prest refresh and I lost it .
I think I may be confused with longitude and latitude.
I section the hair in horizontal bands. I don't actually section vertically although I do work around each horizontal band methodically, overlapping each stroke with the comb.
I can't seem to get the body of the comb (i.e. not just the tips which have no spirals) to contact close enough to the scalp on anything other than the hair sectioned into horizontal bands. Dn't forget that the eggs are usually laid within 1cm of the scalp and IME teh baby lce live on the scalp. Also I often just to a "sweep from the top to teh nape" as a detection. When I find something I then section the head properly and always find loads more on top of what I have found without sectioning (mostly eggs). Sectioning this way also means that you have confidence that you have done hte whole head. Finally - try and actually use the comb at different angles on each sub-section if you can so that you get eggs which are attatched to all sides of the shaft.
At the end of the day. You will find a method which works for you and that is all that matters. It doesn't really matter what I or anyone else does - it has to work for you.
3stars
How did you check your hairs? You need to use a detection comb (The plastic 50p ones from the chemist are OK for detection. Nitty Gritty will detect and treat). You can't reliably check visually
The grit in her hair could be louse poo - it looks like dark sand grains, and could equally be sand grains. You will recognise a louse when you see one. Also you might want to look for the eggs which stick to eh shaft of the hair. They are about 2mm long and the way to tell if they are eggs is to see if yo can get them off the hair. The only way to get live lice eggs off without a comb is to grip them between your fingernails and pull the egg to teh end of and off the shaft. They should be dark so on a babies fine hair they should be easy to spot (uunless she has very dark hair). As for the itchyness that could be the physical crawling or it could be them biting. The bites are often red (and if there are lots of crawlers can be seen visually) and itchy. As the bites can hang around after treatment a child can be itchy for a few days without any actual lice.
I think also shampoo irritation etc. can cause red/itchy head - so it might not be headlice.
HTH