Bald/thin spots in hair ??

[Hayeser]

My 18mo has a lot of hair.  people always comment on how much hair she has.  But on the very top (not back) of her head, she seems to have very thin spots (almost bald).  I don't understand it.  I have to do a comb-over sometimes.    She does seem to have some dandruff or cradle cap junk in that part of her hair.  however, I've read that other moms with kids that have cradle cap don't seem to be complaining that their hair is thin in those areas.

I've tried doing the oil/comb thing.  unfortunately she won't let me do it very much, so I never get to get it ALL out at once.....I just have to slowly work at it little by little. 

what else could be causing this thin hair issue??  ideas??