First check your settings in your amp. Some have a hi/low input selector. If you are using RCA inputs it should be set to low, if you are using speaker inputs, it should be set to high. Next check your crossover points. The sub amp should be set to low pass, then set your cross over point up around 100-120 so you know your sub will register. If you hear it then turn it down slowly to about 80Hz where it should be.
Double check your polarity on your sub. If it is out of phase the gain can be turned all the way up but the bass will be non-existent. Experiment with switching the wires around and see if that helps.
Another idea. Check to be sure that your box is airtight. If not, you will lose bass.
That leads me to an additional thought. Is your box built to spec to match your woofer? Ie, if your woofer requires a box that is .55 cu ft, but your box is too small, the bass will suffer. If it is way too small it can be non-existent.