Hi Lisa...I experienced the same situation with my 9"awning. I installed the awning rafter receivers under the awning bag (see Greg's excellent photos ) but the arms were too long when fully extended. I solved this by drilling new holes for the detents ( the little ball on spring thingies) a little further up the arm. The detents slide out and can be re-inserted easily. One caveat...make sure you drill the hole for the new spot in exactly a right angle so that both detents engage .
To measure for the new spot...set up awning, engage rafter fully, mark the edge of the inner rafter where it exits the larger channel,then re-mark it where the detents engage. Remove the inner channel. The difference between where the factory detent holes are, and where your new holes will have to be, is the distance between the two marks.
Good luck. Remember the carpenter's adage " Measure twice- cut once"