"Pray in the Spirit"
Have you ever had a conversation with someone and you know they weren't truly listening. It's a really frustrating experience that makes you feel sad because you aren't being heard. I imagine God feels this same way when we pray while we are distracted or simply because we are suppose to. At these times it is just empty words -- there is no heart behind our prayer or praise.
I believe God hears all of our requests and all of our prayers.
Psalm 116:1 -- "I love the LORD because he hears my voice and my prayer for mercy."
However, if we are not in the Spirit, if we are not actively seeking to communicate and praise Him, it is empty. When you pray, mean it, don't just use empty words. Give your heart to God and He will respond.