Don't worry, you're not the only one. While I still sell pretty well, I've been steadily declining since I peaked in November of 2011, even with the release of a 3rd book. No particular reason, btw. Good reviews, etc. I was obsessed with checking reports then - not so much now. I keep fearing I'll drop into obscurity - which I guess should be a push to keep writing and definitely to start a different series.
It is summer though, so try not to worry about it too much. Hopefully things will pick up again in September like they did last yer..