why won't my imessage go through?

Sometimes when i try to send a text, it's blue, which is imessage and it gets held up. I have to tap it twice and send as a text message. I can't figure out why this is happening. I've checked all my settings and everything is correct. It happens iphone to iphone and iphone to non-iphone so i don't think that is the issue. Sometimes they go thru and sometimes they don't. only happening since we upgraded the software to 7.