I think what happens if you just set the source to a url is that it assumes it is a vod file and trys to play it as such. You need to tell it that it is a live video and with the current version the way that Richard has outlined is probably the easiest way.
Saying that, Adobe in their wisdom have decided to release the sdk 4 with Flash Builder with an old beta version of osmf which is what the video player component is built on. The current version of osmf is completely different and the video player doesnt even compile with it. Hopefully the next version of the SDK will sort it out.