I am uncertain how to take the faucet head apart in order to clean the strainer and it looks like I need to replace the O ring that you can see when you pull the head down. When I change from spray to stream it doesn't change fully (on stream there are still little "sprays" around the stream.

I just soaked the head in CLR for a few hours. It's the calcium buildup that causes the problem so get rid of that and it'll work good as new.