As stated above, Deftech is known to pad their frequency response specs. That said, it could be you room too. Can you post a sketch of your room, including dimensions, where you sit, where your sub is placed, etc. There are too many variables to say what the issue is. I would start with something simple like the subwoofer crawl to determine if you have the sub optimally placed. You could also be sitting in a null spot. As MKTheater mentioned measurements will be far more telling.
I would expect more from the Deftech also, but it may just not meet your needs. Do you mind posting what you paid? There may be alternate subs we can recommend for the same price or even cheaper.