I go through this ALL the time. Heck, I have sunny skies most of the time and I go through it on a daily basis.
If environment is a key part of your inspiration (which I suspect it is knowing your work) and you feel as if you've been there, done that---then that's probably a good part of it? The other part can be personal-- Whether it's lack of time, exhaustion, life stuff or just needing a new toy to play with the spark those creative juices it all plays a factor for me.
I try to shoot every weekend, but I'm bored to tears with Los Angeles.
So I drive just a tad further, go down another street I've not been on and try to find new spots within LA.
I also try to shift my personal take and perspective on places I've been many times.
I assign myself thematic or creative challenges with each shoot. Or I attempt to go for a particular mood, then I raise the bar higher each time. I think that keeps me challenged--upping the personal challenges and stakes. For example, I can get tired of the same fingerprint I have on my work--so I'll build a new camera and shift around the blur. That always gives me a thrill. Easy -- I am. I even shoot glass to spice it up. Funny, huh? Right now, I find myself jonesing for a nice, little compact rangefinder. The total opposite of my look. But (if I had the camera and the cash at the moment) I suspect it will just help my work grow in all regards. So maybe go out and shoot some uber blur? Some pinhole?
I guess what I'm trying to say is that it's all about playing for me. If the fun is gone--I hit the dulldrums and lack inspiration.
Maybe ask yourself, what would make it fun again for you? What could pose a challenge photographically to you right now? Does that challenge interest you? Heck--then go try it.
One other possibility....When photographers reach a point in your work when your work is continuously exceptional-- like yourself-- it becomes harder and harder to top yourself. If I was to put myself in those shoes, it might become harder and harder to pick up the camera and just play, since I'd expect to be better or equally as good in this roll than the last. That's how plastic can help--you can't place expectations on anything, since they are so damn unpredictable. If plastic is boring you--then find another avenue to help you get past the need for perfection so you can play again. (that is, if that's where the dulldrum problem hides)
Of course, all of these suggestions could be completely off the mark for you, since I'm just coming from my own dulldrums experience.
Hey. Here's a wild hair idea. Try to shoot something "invisible"?
Your fan,
Susan