Why do my subs get quieter and echo less sometimes?

I have 2 10" kenwoods with a 1000 watt kenwood amp i think, i dont know all the specifics. but lately, they have only sounded about half as good, especially on the lower notes. none of the notes echo hardly anymore, and the low ones make some weird sound that sounds like a bullfrog or something. when i jiggle the speaker wires and power wire connecting to the amp, it fixes it sometimes but only temporarily. can anyone tell me why they do this? and how to fix it?