Try different mounting locations. Try it firing forward, back, up, and even down (Spaced 2-3" above floor) in these different locations.
Brace your enclosure. Threaded steel rod works wonders, as well as long thin scrap MDF.
Round the inside corners of your enclosure. Those thin strips of MFD and a concave router bit will do the trick.
Use fiberglass resin to line the inside of your enclosure, this will in turn raise your tuning a little, but some people swear by it. It also ensures an airtight box.
Play with your amp and HU's settings. Even if they end up right where they started, at least you can be confident they are set right.
Finally, know what you want. If you have 60dB of music, and 90dB of bass, and "it's not loud enough" it could be that you're constantly cranking the volume to get the music back. Or, if you lack midbass, you could be trying to use the sub to make midbass, which it's not suited to, and cranking it higher to compensate for the weak midbass output.
$0.02 only. Take it as you will, but I hope it helps.