Several of the older ferals we had fixed always came and went - very few were here "all" the time, like the kittens and their families we cared for. The one family we were closest to - mom cat disappeared completely after we had her spayed. Never seen her again.
The others all came back after a few days. I think they get a little freaked from being trapped and the vet visit - and then come back for the regular, easy food.
And if you keep putting out the food, they will come. If not the ones you had fixed, then others. Once you start caring for them, Katie's right! It's like hanging out a neon sign, and the "cat network" swings into gear.
The hardest part of caring for ferals is when you don't see them. We're always wondering - did something happen? A fight with a raccoon? Hit by a car? Poisoned by one of the farmers around here? The waiting to see them again is THE WORST. I know EXACTLY what you're going through!
Keep your hopes up, send up a few prayers or "kitty come back" vibes....
(and this is why we always wait until we're sure kittens are 12 weeks old before trapping mums to be spayed. At this age is when she'd send them packing to be off on their own anway. So just in case mom doesn't come back after being spayed, we want the kittens to have learned all the need from her....)