Yours is a serpentine belt, right? Mine are v-belts so I can't help much. You checked the bolt on the crank, right? It has to really get torqued on there, like to 200ft-lb or so. If it's loose, maybe the crank wheel is out of line? There have been some instances of broken woodruff keys on the crank nose too.
The A/C compressor on my car cannot be adjusted longitudinally, but I did note that the stamped bracket is a bit twisted - I haven't straightened it out yet but I'll post some pics when I do.