Subs not hitting hard after a day..?

Hey guys, the other day I got some subs, not a great pair just a beginning pair just to start out, it was like $200 from pep-boys. It was SDX audio, and it was one where it had a built in amp, and it was working perfectly for like the first hour, and by good I mean like my entire seat was a massage chair while listening to Mercy by Kanye West(Not the best song I know), then it just stopped hitting that hard while I was driving. It still is working cause I can turn the stereo on and like feel both subs moving but its not hitting hard at all. The speakers pre-installed in my car have a better bass then it.

Can anyone figure out why this could be happening?
Any help is much appreciated! Thanks ahead of time!