Subwoofer works at high volume, doesnt work at low volume

Just recently noticed that my sub only works when i turn the volume up pretty loud or i turn the bass up on my alpine deck higher than normal. When I listen to music at low volume levels, the sub doesn't seem to be working. Also as of today, I'm getting nothing out of the sub even at high volume. I checked all of the connections from sub to amp and so forth, but could it be something inside the sub box, or maybe is there a fuse out??? Thanks for your help.