Yep, we have had the same problem, and it is a fairly common problem. One of the frequent failures we see is when the owner uses air to purge his/her lines for winterizing this is the opportunity to blow some "junk" into the seat of the check valve rendering it incapable of flushing itself out and no longer working. It's kind of a catch-22, if you don't blow the lines you will not remove all the water; if you do, you may fail the check valve. One thing we suggest to all our customers is to keep the water heater flushed well and you eliminate a lot of the source of the "junk".
It's located in a pretty akward place too isn't it? Ahhh, the fun of tinkering.
Mike & Lisa
2007 Tiffin Phaeton MBE,US Gear tow brake
Master Certified RVIA Tech
Ford Ranger toad (worktruck)