the two things that make sense off what you said, higher than ideal FG and that's a low IBU for an APA. Try holding your fermentation at 64 like you did for 3-4 days then let it climb up to about 70 degrees to finish it off. I learned my lesson with an Imperial Brown, the airlock was going for over 2 weeks before my gravity readings finially leveled off. Letting the temp bump up over time will get it to finish lower. The other is definitely throw some more bittering hops at it. For a beer at 1.058, you're looking at at least 40 IBU's to start to get some hoppiness to it. Good luck on the next one!