I had a little one who stopped putting weight on between 3-4 months and was seeing a pediatrician for this. He was never concerned with what percentile her weight was in, only that her weight gain was following the curve.

When it came time to start solids, I asked if there was any medical reason for not doing BLS (my MCHN was not really going to be supportive of BLS). Aside from the issues mentioned in Gil Rapleys book, he said only if she stops putting on weight. Milk remained the main food for her. Her weight gain since has been great.

Good luck!