Not a lot to go on here, but I'll chime in with what I know and try to answer the question.
Medically we don't know what is wrong, and there may not be anything wrong. My daughter was diagnosed as "failure to thrive" and had a g-tube placed for feeding.
In addition, she had bad acid reflux which resulted in lots of vomiting. She didn't gain a lot of weight, especially once she became active and learned to walk.
She's 3 now and has been non-stop. We hear comments all the time that she is "tiny" or "small for her age." She is on the lower end of the growth chart, and even though she now eats orally, her combination of high activity level and at times picky eating has kept her weight low.
That's the TL;DR of my experience. Now the advice.
Therapy. In the US anyway, we had access to both speech therapy and occupational therapy to help her with her feeding issues. Life saver. Literally. We were able to get off the g-tube in part because of the therapy she received. One day she decided that she would eat and not give up. Prior, she would eat a few bites and be done but we couldn't really let her go hungry either because we needed her to gain weight to get off the tube. It was a vicious cycle.
Milk and butter. Her GI specialist was great and understood our struggle (this was his area of expertise); his main recommendations were to get her to drink lots of milk and put butter into her other foods. Even when she had the tube removed, his only concern was that she get plenty of milk. With that said, its difficult to get kids to do anything they don't want. She wants milk when she wants it, so when she does we let her have as much as she wants. I wish we could get her to drink more as I'm sure her doctors would be more impressed, but hey, it is what it is.
Eat when she wants. But within reason. We try not to give her too much if its close to meal time, but in between we let her graze on just about anything. It might be a bit of bad habit, but for us we are just happy that she does eat orally. It was easy early on, when she was on the tube, to give her anything because we knew she wouldn't eat much and she'd get her tube feeding but now we have to be a bit more careful so that she will eat her proper meals. One thing that she still loves, a hold-over from her tubie days, is apple sauce. I thinks its become her comfort food. It was one of the first foods she ate.
Some nights she doesn't eat much of her dinner and others she eats more than enough. We don't let her go to bed hungry though, so we try to give her something even if she doesn't finish dinner, can't blame her if she doesn't like everything. But we don't just give her junk food either. And sometimes I think she just isn't that hungry.
I would make sure to get as much information as possible from the paediatrician and/or a second opinion if you feel there is something else wrong. We asked a ton of questions. And we didn't think we were ever getting off the tube either, but while her progress was slow, it was stable and it was progress. That's what they like to see. Weight loss is a much more serious problem.
Hope that's been somewhat helpful.