how do i increase kitten appetite?

i have 2 mixed-breed persians and they're going 4 mos. last time they're weighed they were around 1.5kg both and i was told by the vet it was healthy for their age. however the female kitten got sick recently (upset stomach, she's okay now) and she visibly lost weight. the male kitten also seems to be having less appetite just like his sister.

what can i do to help them gain weight? they have 2 vitamins given by the vet and are being fed Whiskas kitten wet food, dry food, and boiled chicken.