Do you mean actually fade colour wise or aren't as "bright" as they first were? The brightness of most colourant treatments fades within a couple of weeks, but if you mean your hair is actually darkening then that's a bit different.
Do you use a shampoo for highlighted hair, a blonde one with camomile in it maybe?
I have my hair highlighted too (my hair is naturally dark blonde) and they last for ages- they hardly fade at all. I use a special shampoo for highlighted hair, I use VO5 Fade Defy(red bottle) and that seems to keep the highlights looking bright and fresh. Perhaps speak to your hairdresser next time you go and see what she says, if there is still no improvement then perhaps find someone else to do it for you and see how you get on.
I have highlights in my hair too. The highlights weren't as bright after a couple of weeks but they still look good and quite bright (not much differenc) even though they are starting to grow out. I just use shampoo for clour treated hair and then now and again I put my hair in some sort of mask/conditioning mask.
I use Sunsilks for blonde hair shampoo and conditioner. It's for either natural or highlighted hair. It has chamomile in it, which is great for blonde hair. My hair colour is natural with no highlights etc but I really recommend it to you to try. My hair has been so much lighter; more like it is in the summer by using it. If you do try different hair products I would advise that you steer clear of Clairol's for blonde hair shampoos and conditioners because it burnt mine and my friend's scalps off- it was really painful so you definitely don't want to go there!
Wella does a purple shampoo that I got from my hairdresser and whatever the purple is in it cleanses the bleached bits and stops them from going dull, not sure if you can get it in Germany though.