PCB etching not working: everything etched
As already described in the comments, the develop solution appears to be too strong. 

The traces should be nicely visible when developed. The fact that you saw them appear suggests that your exposure was at adequate. Too strong developer will eventually destroy even unexposed areas.

I am using similar boards, with granulated NaOH (sink cleaner) in concentration of 7-15g NaOH per 1 liter of water as suggested here.