Are you tag spamming? This video is about tagging video, but their advice is great for blog posts, too. Check this out:
So, don’t go nuts. Use the primary keyword tag, for sure, and use it in your title, description, and in your tags, but don’t go nuts. Use a few tags that make sense, but not every tag you can.
I’ve heard advice given both ways, but this makes perfect sense to me. (And now with Google cracking down on so many things, I’d take the advice.) It’s like keywords. If you use too many, it’s tag spamming. If you use too few, people won’t be able to find your video or post. So, choose around 5 really good ones and make those work for you!