Klipsch subs are not really known as great performers. But that's not your problem here. I would always recommend multiple subs as it will even out your bass response in the room. If you want to take your experience to another level, step up to the better quality Subs from SVS, Hsu, Outlaw, and Monoprice Monolith... also Subs that get mentioned a lot are Rythmik and PSA.
To your problem, acoustics are a b!tch. It could be as Bill suggested, you could also be getting a cancellation due to the soundwave reflecting off of something and bouncing back to the source (so you are looking at a reflection between 12' and 14' away, or half of that.) Just a possibility... and there are many.
How did you select placement? If you did not do the subwoofer crawl, I highly recommend it. It is a crude technique, but it will teach you a fair amount about how Low Frequencies behave in your room. If you do it, look for the best 2-3 locations where the bass sounds best, and listen for the subtle differences you may hear as you move around the room. Keep an open mind about potential homes for your Sub(s). Getting the best performance doesn't always mean corners or front wall placement.
Do the Subwoofer Crawl. It's the easiest way to find the proper placement location for your subwoofer to get the best performance and sound.
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Other things to try:
Move your seating, subwoofer, and or speakers slightly. Inches can make a difference. You could just have you seat in a bad spot in the room, for example. My system was set up symmetrically until I discovered a problem, I moved one speaker by about 1.5" and changed the toe-in slightly to fix it. Now my mains and Subs are each different distances from each other, and from me, and in this asymmetrical arrangement, I found better response.
As Bill asked, please give room dimensions. Photos can be helpful, too. Also, figuring out the actual open room volume for your subwoofer... l x w x h... how much space it has to work to fill... is important.
Cheers!