Well the male looks really old and with my close observations of the guppies i use to have that reproduced. I had 3 males 2 were smaller and younger, and there were ALWAYS tryin to use their weapon on the ladies if you know what i mean, and the older bigger one with bigger fins was never really interested in the ladies....as if his sex drive was gone or something. So thats a possibility. You might want to get a younger male.
other than that, everyone else said it all, temp up. And you'll DEFINATELY be able to tell when shes pregnant.