There is much more to FTT, especially at that age. Even with a tiny one like DD (4 weeks onwards), it took months before they actually put FTT (with a ?) on a piece of paper. One dr threatened to do it though, some of them use it to bully parents.
There is a definition, even if I don't remember it (and DD NEVER fitted into it). It does talk about output, weight gain, development, strength and possibly more. If weight is the only issue, the medical def. is Slow Weight Gain, , and that is usually ok, a lot of babies just gain slowly!
I would just offer her food, if you stress about it, she's more likely to not want it! Just play with food. It took quite a while before DD was eating much solids, she was probably about 16-18 months old by then. And she could, and wanted to, feed herself from 6 months, she just didn't actually swallow that much of it until later.
And weight gain slowing down when they get more mobile, I thought that was the norm, most babies I've known have done that. Growth at 8m+ is very often in stops and starts - all of a sudden she might gain quite a lot!
Also, head circumference is most important, length comes next, weight is just the easiest indicator that something might possibly be wrong - any idiot can weigh and chart a baby (sorry that is for people like your ped, like our first ped and our first nurse).