If they only came yesterday, I wouldn't worry too much, but leave food out overnight, you will probably find that if they are in the habit of only coming out at night, that is when they will eat. PEbbles was fussy at first, but her plate was always cleared overnight (I know they say not to leave wet out that long, but at first, it was the only time she would eat it, and at this time of year, there isn't as many worries with flies.)