I wouldn't say it was a "problem" per se, but there are times where I manually turn them on. I have had some rain storms where it just wasn't "dark enough", but honestly, I manually turn my lights on anytime its raining - sun or dark. Just helps others to see me so I do it out of habbit. It works very well everywhere else tho, just like you stated. I don't think you have an issue, it just depends on how much light is hitting that sensor to tell if it's "dark"
It's normal in my opinion.