I don't grow this species so I don't have much to offer. Are these not more suited to warmer locations? Perhaps the lack of growth is related to cooler temperatures than it likes and/or less light at this time of year.
Your watering sounds good in general, but again I can't say specifically. Not sure why you would be getting mold due to fertilizer. I have had that only when I used a solid organic fertilizer. What are you using? You can always scrape it away with your fingers or an old toothbrush. At any rate, I don't think that the mold is the casue of the problem
Can anyone else who grows Bougainvillea help our new member?