Chrome extension content scripts not injected when navigating

Question

I'm developing google chrome extension and encountered following problem. Content scripts are not being injected when navigating on YouTube. However, when the page is reloaded or opened in a new tab, they work. It seems to work on any web site except Google's ones. No errors in console. Tried different "run_at" options with no luck.

manifest.json

{
    "name": "Test",
    "version": "0.1",
    "manifest_version": 2,
    "description": "Test",
    "content_scripts": [
                        {
                            "js": [
                                   "contentScripts.js"
                                   ],
                            "matches": [ "*://*.youtube.com/watch*"],
                            "run_at": "document_start"
                        }
                        ],
    "permissions": [
                    "*://*.youtube.com/watch*"
                    ]
}

contentScripts.js

alert("TEST");