Guppy not eating

I have one female guppy that nibbles at food but always spits it out. All
the other guppies are attacking the food like sharks. This specific fish
doesn't hide, but hardly moves compared to the other, quite active, gups.
It doesn't appear to be injured and it doesn't appear to have been
chased/picked on by other fish. Also, it doesn't appear to have that
"gravid spot" that indicates pregnancy.

It used to be as active as the other fish. Not sure what brought this on.
Very new to guppies though.

Its in a group of 8 guppies and 2 small goldfish. tank temp 80deg F (heat
wave in these parts and thats the best I can do), pH approx 7.1, Amm. approx
15ppm, 29g tank, with live plants. I've had the guppies for about 2 months
and the goldfish much longer.

Any advice,
E.Otter