Do you have any other fish in the tank and could you post a picture of the setup? It might help to add a few cory's, seeing them eat might stimulate the zeb's to start eating too. Would also consider darkening the tank as much as you can, this will make them more at ease and reduce stress. Wouldn't ...

Here's my setup, works great for me..

http://img.photobucket.com/albums/v136/Rainbow007/IMAG0727.jpg http://img.photobucket.com/albums/v136/Rainbow007/IMAG0725.jpg http://img.photobucket.com/albums/v136/Rainbow007/IMAG0722.jpg http://img.photobucket.com/albums/v136/Rainbow007/IMAG0716.jpg http://img.photobucket.com/albums/v136/Rainbow0...

signed.

Sap, the first fish looks slightly deformed to me, did you notice the odd fin shape?
Not sure about the first fish, the second and the third one look like males.

Second pic from left to right:

Row 1: ?, F, M, ?, F
Row 2: M, M
Row 3: M, ?

Some better closeup pics would be very helpful to give you a more conclusive idea though.
On a side note, great looking fish of a decent size, I thought your fish would be much smaller.

Welcome Brecht, nice to have you here

top to bottom

female?
male
female
male
male

top to bottom

female
female
male
female
No clue.

Agree with zeno, all males.

the first one and the last one are female, rest are males imo.

top to bottom

female
male
male
female
female

These are in great looking specimens Sandy, the 4th one is a female, the 5th one could also be female imo.
How old is the last fish exactly and what's the size? I'm pretty sure the first 3 are all males.

Sadly the vid turned out very dark uploading it to youtube, but I'll still share it as it's pretty unique footage.

Trapping:
http://www.youtube.com/watch?v=-7eV1jUJBMo

A short one:
http://www.youtube.com/watch?v=w7zNQZxEeC8

I keep my adults at 28degrees, they also breed here on this temperature.
The tank with fry is kept slightly higher at 30 degrees.