new pup but older dog not eating - Page 1

I have recently introduced a new gsd male pup to the house to join my 2 bitches aged 12yrs and15 months respectively. The 18 month old bitch has never been a great eater and has always picked at her food at the best of times. She is a tall dog but slender who is very fit and active,however i feel she is too thin and i am constantly trying to make her gain a little weight.
Since the pup arrived she has eaten next to nothing and seems really out of sorts, she is still active, drinking well and playing too but very unhappy and no appetite. All the dogs are fed on arden grange, the eldest has senior, the 15months bitch has prestige and the pup is on large breed puppy.I am hoping to not have to start changing feed as i find that they get picky when too many choices are introduced but i was wondering if the recent hunger strike could be due to the new pup, i would welcome advice and suggestions on this. All dogs are wormed correctly and see vet whenever required but i think she seems depressed!!! please help.