Twitter4j: Refine search with hashtag return empty result

Twitter4j: Refine search with hashtag return empty result

I'm using twitter4j in my application to search tweets and then filter the
search results with a specific hashtag, but I have a strange issue.
When I make the first search, I get a list of tweet and parsing this list
I get a list of hashtags also. Now if I try to refine my search adding to
the initial query one hashtag of the list, a lot of times I get an empty
result list.
For example (I'm sorry but I have just in italian), if I search "canone
rai" I get a list of tweets and a list of hashtags, which contains "#tv".
If i add the hashtag to the query I get an empty result. How it's possible
this behaviour??