They retweet things they see under that tag for that day, but Twitter doesn't show all the tweets for each tag when you search, so it's possible that they're just not seeing your work, especially now that Pixel Dailies is quite popular. Twitter also features popular tweets more prominently and doesn't dump them as readily, and popular artists get likes and retweets even without PixelDailies' help, which might explain why those artists are more consistently retweeted by PD.