I also have XM issues. I installed a non-Harley XM radio on my 2005 Ultra and had excellent reception with the antenna mounted on the top of the radio as close to the front edge as possible. It worked very well.
I have had issues the my 2010 Ultra CVO with the factory XM installed. The signal was lost frequently, even out in wide open spaces. I removed the original antenna (mounted under the windshield trim) and replaced it with another antenna mounted, again on the front top of the radio. Signal is now almost always three bars strong. Even with three bars, the radio becomes silent quite often. This weekend, while on the America's 911 ride to NYC, the XM stoped working completely. Had three bars reception, but the message continued to say "LOADING" and never did start working. I shut off the radio, back on, and off several times with the same results "Loading." Later in the day, it started working again! The HD XM radio's SUCK! I have two other vehicles with XM and have no such problems.
I'm going to tell HD to remove both the the XM and Navigation modules from my bike. They are both next to useless to me. I'll mount my own XM & GPS units like I did on my previous bike.