It's not a 15 day service, it's a priority service that is treated ahead of other applications with no offer to guarantee a finite
prazo. There may be problems with the application (something about your partner's circumstances that needs to be investigated) or, if it is genuinely straightforward, there may just be long delays due to technical or logistical issues. I would have imagined that someone paying for a priority service would be sent a courtesy email saying that there are delays and giving a reason, but that does not seem to be happening.
http://www.immigrationboards.com/viewtopic.php?t=102474