Sable is Small and has no Appetite

I got Sable two weeks ago, I was told she was 2 months old.

She weighed 3.4 kg at the time and as of 5 days ago was 4.1 kg. I'm worried as she rarely seems interested in food.

I've been trying to give her Royal Canin maxi junior but she will hardly touch it. For a while she enjoyed it when mixed with some Pedigree chum wet puppy food or some chicken. After a few days she got bored of that too. If the dish is left out she will eventually eat a small amount but for two days after vaccination she barely ate a thing, maybe two or three nibbles but probably in the region of 20 grams or less.

I don't want to spoil her by pandering to picky eating but my main concern is her weight. I continue to give chicken a few days a week and am also giving her nutri-gel as a supplement.

There is a chance she may be part Vietnamese dog (I live in Hanoi) although i was told her mother was full GSD and part of a pack of 4 who the owners breed.

She still has a lot of energy and playfulness so I'm wondering if she might just be younger than I was led to believe.