I have a 17 month old Malinois, Heidi who to be honest has never really been a big eater. She  often eats a meal and then picks at the next 3 or 4. She used to be fed Bakers as my Lab was but when we got our new puppy she was on CSJ so I changed them all to it. About 8 weeks ago Heidi was weighed and was 20.9kg (quite low then I thought) but we noticed about 2 weeks ago that when you stroked her you could feel her spine. I took her to be weighed and she was 19.8kg. I didn't book her in to see the vet as she hates going anyway and I knew he would say she was losing weight because she isn't eating. I have since brought about 5 different sorts of food, some she wont touch, others she has eaten a little of (James Wellbeloved and Royal Canin) but she still only eats a few mouth fulls and then walks off. I know people always say that a dog will eat it when they are hungr enough but she really wont! She is a very active dog and gets walked for at least 1 and a half hours a day off lead and does agility training so it isn't that she isn't exercised enough. I know the heat probably hasn't helped over the past few days but i'm quite stuck now. Some advice is greatly needed and I am going to make her an appointment with the vet for next week.

I would take her to the vets for a check up first even if she doesn't like it.
Some adolescent dogs especially active breeds do go through a thin leggy phase.
Have you tried actually weighing what she does eat? it might help reassure you that she is getting something. Look at training treats as well if she gets some of them it might mean she has less of an appetite for her meals. have you tried working her for her meals using a bit of her meal as a reward? sometimes dogs who have little interest in a bowl on the floor will eat if given as a reward for doing something.
Also what about a bit of competition. if you have other dgs will feeding them nearby encourage her to eat to stop them getting the food?
My older dog was a very poor eater when she was young. She would eat one type of food for a while and then stop, and then another. I added tinned meat etc to her food and it would help for a while but then she would stop again. She is the reason I now feed raw as I tried it out of desperation because she just wasn't eating much at all and she loved it. Part of the problem I think was boredom with the food. with raw she gets different meats at different meals so they are more interesting. I also found the volme of meat needed was less than the volume of kibble so she coule get what she needed in smaller amounts.
I know raw is not the easiest way to feed and not everyone can do it but it certainly helped my picky girl.

Another thing I have just remembered, Hormones played a part too as she would gain and lose weight at different times in her cycle. Trying to remember now I think she ate poorly after her season and then much better coming uo to the next one, but it might have been the other way round. She is spayed now,  and since she has been spayed she seems to put on weight at the drop of a hat and I have the opposite problem of trying to keep her weight down.

Thanks for the advice, we got some raw food today and tried it tonight and she just turned her nose up at it. I've tried feeding her with the others and alone, neither works i'm affraid. We are off to the vets with her on monday as we really are out of ideas now, its been 2 days since she has eaten anything and i'm begining to worry about over exercising her now with no food intake. We will have to see what they suggest!

B Sorb is good for keeping dogs going if they refuse to eat & it also acts as an appetite stimulant. I would still take her to the vets for a good check up