Hi- I am the mother of a "peanut" as well, my daughter is 14 months old and 16 lbs 7 oz. Like your daughter, she is also a very healthy and happy girl, meeting and exceeding all other milestones. In addition to making sure there isn't an obvious underlying problem, through the tests your pediatrician might make- it is good to get some tips on feeding strategies, too. I found that my pediatrician wasn't too interested in getting much info about what exactly she ate, how much, how often or when- which I thought was strange. And overall- he never was concerned about her being small until her growth curve dipped a bit in percentiles between 10 and 13 months- that it was ok as long as she was consistent. And they never really took into consideration that my husband and I were both really small babies/kids for a while (I am 5'8" and husband is 6'2").
But enough of that- I know how frustrating it is to feel helpless when you are doing all you can to promote weight gain for your baby. Here are a few strategies that are working for us (we had good weight gain at our checkup today!)
1. you know this already- but of course the avocados, bananas, whole milk products and soft pasta (soft is easier to digest than al dente), and meats.
2. snacks- a couple of snacks per day, in between meals may help to fit in more calories- crackers and cheese, grapes, cereal and milk.
3. We have had success by feeding her "2 meals" per sitting, instead of just one. She used to just eat each meal with us- but now I serve her food first so she gets to eat as much as she wants- then she gets down to move around a bit, then we all sit down to eat. The food settles a bit, and she is interested in OUR plates of food so she then eats more.
4. Whenever my daughter gets pokey, or starts to refuse food (if it happens early in feeding), I don't make a big deal of it but just do something else nearby for a few minutes, or prepare food or snack for myself. Or offer her a sip of milk or juice/water to help her swallow what is left in the mouth. She usually asks to then be fed.
5. sometimes I feed my daughter while she is "on the go." I don't know how safe this is to do, in regards to choking hazards, but sometimes she really just needs to move around before she can eat more. And I stick to soft, pureed foods if she is moving around while eating.
6. If she refuses one thing, I may offer her something I know she likes- and then sneak in spoonfuls of the other things in between bites- works for her!
7. I load the spoon and then let her hold it, or just let her spoon it herself- she loves to feel more in control, and will eat more when we do this.
8. Lastly- check out "Happy Baby" frozen organic foods- they are really dense in calories (especially that baby dhal one) and very tasty- my daughter loves to eat them straight or topped on whole grain bread.
Hope this helps, it is an issue I have also spent a lot of time with.