How long has the tank been set-up?
Black Widow Tetras are fin-nippers so they may be nipping the Guppy and pestering the Corys.
Neon Tetras often do badly in newly set up tanks and usually die because of that.
The Corys should be in a bigger group.
Which species of Corys do you have?
WHat species of cory do you have?
I would do this stocking for now:
6 Neon Tetra
6 Corys, either 6 albino or 6 Bronze Corys or 6 Pygmy Corys
4 Male Guppies
And finally 2 Honey Gouramis 1 male 1 female
In total.
I definatly remove the Black Widow Tetra
Black Widow Tetras can fin-nipand they are bigger and chunkier than say...Neon Tetra... and need to be in groups of 4+, therefore, they take up more room in your aquarium, leaving less space for other fish