The best thing to do is return it to H&R, the Sportsters seem to be notorious for this problem. Contact H&R and ask them to cover shipping, toll free (866)776-9292. You may be able to fix it using Fred M's recommendations, although his is for a centerfire, should be close enough, it would require and entire R&R of the trigger group, click his website in his profile, Perklo's trigger R&R can be found in the FAQs.
Tim