I've posted this on another thread and this is how it is:
A new user or someone with not many followers tweets or retweets with a url from a non authority website, that tweet wont show up on twitter
If they same account tweets or RTs with an url from an authority website ( any large site like BBC, YouTube, etc...) then that tweet or RT will show on twitter search
So, the difference here is the url in question. URLs from non authority websites get ignored if they are tweeted from a new user or someone with not many followers.
Not very fair when someone might want to get new followers interested on a same topic but since their tweets won't show up on twitter then it's hard