Pair won't breed

[Todd Brabbin]

my 2 3-month old gerbs can't seem to figure out how to breed.  they are
definiately male and female.  they get along great, play together, groom each
other, and sleep together.  she just doesn't get pregnant.

does it happen that sometimes they just don't breed?  Anyone have this happen
and then just one day it happens?  Has anyone had a pair that never bred?

Just seems weird.

Todd