Unfortunately without examining your saw I can not be certain as to why you are having a problem. Please have the nearest DEWALT service center evaluate your miter saw in order to determine the best course of action to correct this issue.
A list of service centers can be found on our web site, http://www.dewalt.com/us/service/center_finder.asp or if you need additional assistance in locating a service center please call us at 800-433-9258, Monday – Saturday 8:00 A.M. – 5:00 P.M. Eastern time.

You can remove the adjustable, Sliding Fence by backing off the retension screws and sliding each side of the Fence off of the Base. This will enable you to use the Base's Back Fence for larger stock. To crosscut a 2 x 8, you can add an Auxiliary Base to elevate the stock -- so that it's level with the Back Fence. You won't need to purchase an additional base, as you can use a Sacrificial Base. That is, a piece of scrap lumber that elevates your 2 x 8 or 2 x 10 to a height that will position the larger stock for the 45 degree cut (2 x 8) or 90 degree cut. (2 x 10)
You may want to view the video in the Product Virtual Tour, as it displays the DW716 Miter Saw using a Sacrifical Base that is set-up to cut a 2 x 10. The image (Copyright DeWALT) displays a frame from this portion of the video.