Highlight w/roots not covered

My daughter is 14 and has had her hair highlighted for about a year. We recently moved and have been going to a new stylist for about seven months. She has highlighted my daughter's hair, which is naturally dark blonde and just past shoulder length, twice, using foils. The color is fine but it seems that she is not getting the color close enough to the roots on the hair that is lightened- about a 1/2" of the darker color shows through and looks strange, especially when my daughter wears her hair back, which she does often. It looks like she needs her roots touched up when she has just had it done and the highlight is barely lasting five weeks.
The stylist corrected it for us the last time with no problem but I almost hate to call her again. I don't want to seem like we are perpetually unhappy! I am afraid to do it myself because I don't want to really mess it up. But I don't want to go somewhere else to have it corrected and be charged again.
I have noticed one difference in her technique from our previous stylist - she places the foils on either side of my daughter's natural part while the previous stylist put them right down the center on the top of her head. I don't want to presume to tell this lady how to do her job (and she is very sweet so I'm not worried she will get huffy), but would that make a difference? I have never had this problem before and I have had my own hair highlighted for at least twenty years. I can't afford to have my daughter's hair highlighted every five weeks - and in my own experience, I shouldn't have to. I also don't want to have to keep calling to voice our displeasure. My color and our cuts are great; it's just this highlighting on my daughter that's the problem. Any ideas/advice would be appreciated!

Thanks for your response. I am going to call our stylist today and talk to her about my daughter's hair. I thought we were on the same page after it was corrected the last time, but I guess I was wrong. Even my husband noticed that her hair was considerably darker in the front (and that's a first!).

I will probably ask around about other colorists; it's hard when you are new to an area.

My daughter went back to the stylist yesterday. She put the foils right down the center of her head, instead of on either side of her part, and it looks much better! That's the way it will have to be done for her, I guess.