I just posted elsewhere that my youngest, who has tested as pancreatic sufficient and so is not on enzymes, is 23 months and 24 pounds. And that puts her at a better BMI than she has had for a while, but nowhere near the 50th. At one point, she was about the 2nd percentile. She went for about 7 months without gaining a pound. Then she put on 4 pounds in 3 months. Go figure.
What we focus on in the overall trend in her BMI. Is it heading up? If so, then great. We keep working on it. If it is heading down, then we start to watch more carefully and supplement her foods more aggressively. But the others are right; at this age, a child grows and gains erratically and at his or her own pace.
We do everything we are supposed to with adding calories. We put extra pats of butter on everything... she LOVES it, and it has helped pack more calories and fat into every bite. Our nutritionist recommended dried fruit once they could chew well enough, as that is a great source of lots of calories that don't fill you up. It has helped, and my oldest has a BMI above the 50th, but the little one is still, well, really little. It takes time, and it takes perspective to reassure yourself that you are doing what you can do.
In the end, remember that we can't MAKE a toddler eat more food than they want to eat. All we can do is give them high-calorie, healthy food options. Sometimes those in the medical fields forget what it is like to live with this every day. They forget that when they say your child has not gained enough, we hear we haven't done enough, and that that message hurts like hell when you are doing everything you can to give your child the best outcome you can.