Agressive bristlenose

I have a 15 gallon planted tank which I set up about 4 months ago, the first fish to go in the tank were 8 neon tetras, followed by a small female bristlenose catfish, 2 weeks after adding the bristlenose I decided to get a male to keep her company, knowing I would soon be upgrading to a larger tank. since day one of introducing the male bristlenose the female has been extremely aggressive towards the male, if she gets sight of him she will chase him around the tank, away from food etc.
It's been afew months now and the fish all seem healthy but the female is still aggressive towards the male, and she is growing at a faster rate than him probably because she is dominant over the food.

Has anybody got an explanation for this bahaviour? The only thing I can think of is not enough room for 2 of them in a small tank, there is plenty of driftwood and plants, but maybe the female thinks she rules the place since she was there first...

I have now setup a 40 gallon tank and have transferred the male into this tank on his own so he can get some confidence back and get a good feed all to himself to try put on a bit of size before I introduce the female and the neon tetras to the tank with him.
Hopefully the female will leave him alone this time since they have alot more room now...