3'/4' JDM lowering kit suspension problem.. Please help

I have an 01 Silverado Ext cab. I bought a 3'/4' JDM lowering kit. I had a shop put it on, and when I was driving home I heard a loud bang. The driver side dropped, about a half inch. I took to a few mechanics noone can figure out what the problem is. One said maybe my coil springs are warped. Don't know if that is the problem. The only thing that is not new on the front end are the coil springs. I still have the originals there.  I went and replace the springs today and it is still the same.