Loud squealing noise when I start my car

When I started my car it made a loud squealing noise so we replace the Serpentine Belt, the noise stopped for a few weeks, but it started squealing again the other day this time its alot louder then it was before, when I accelerate the noise gets louder, when I let off a little its not as loud, what could be the cause of this and how do I go about getting it fixed. Also when im looking down at the crankshaft and harmonic balancer it looks like the center hub that the belt goes on has moved. Could the belt be lose and is the reason for all of this, or could a pulley be the cause of this. Thank you for taking the time to read this and any help would be great.