Hiya hun
Normally dry food is quite fattening and esp indoor cats on a mainly dry food diet are prone to be over weight.
Did the vets do any checking for any other reason for her weight loss?
Is she drinking water ok? Very important with dry food for them to have plenty of water or it can lead to kidney troubles when older.
Wet food does have much more moisture so does she like any wet food ?
Try her also with chicken and white fish.
I give mine a mix of raw, wet and dry.