IME, the female is less inclined to spawn if the male isn't considerably larger than her. I think its down to her wanting to achieve the best success rate with fry (better protection). But if you provide a piece of slate or smooth rock angled at 45 degrees and do some cool water changes you will encourage spawning.
On a side note - from the picture i think it is possible you have two males. Also, if you got a male and female both at 2.5" the male would have more than a quarter of an inch on the female. I got 5 at 2" in december and the male is now over 6" and the female nearer 3". So the growth rate varies massively. Tying on with what blue phantom said, about strange behaviour, i have never had my pair liplock, nor have i had the female show to the male. Its always the male following the female and flirting.
Not saying you definately have two males, but it may be something you might want want to look into and consider.