Why is sexing retf so hard?

[Gomugomuking]

I thought male at first, but then again it's been sick since I got her. And although he/she's getting better with the right treatments, he/she still doesn't eat much or move around that much. He/she hasn't croaked at all, unlike the other one I got, who croaks all through the night. So either it's a boy and isn't feeling well enough to croak, or it's a girl and isn't feeling well enough to fatten herself up. Though he/she is getting fatter by the day. Anyways thanks, I hope he starts chirpin' soon so I'll know. Here is a pick got while it was climbing up the wall when I got home.