This may be normal, because breastfed babies do not growalong the same growth curves as formula fed babies, and it may appear that they grow too slowly, when in fact, it is the formula fed baby who is growing too quickly.
Children growalong their own curve, which is why a baby who is in the 5th percentile and has always been in the 5th percentile is less of a concern than a baby in the 50th percentile who suddenly starts to drop.