3M has created a nice article to detect bad impressions and the root cause behind them. Its a nice little read and very helpful if having problems with impressions and/or crown placement.
All these articles are designed to help you identify common problems that could occur in impression procedures, Impression Tips highlights an issue, discusses potential causes and offers solutions for improving impression results.
Bite Registration (pdf, 186K) Double Bite Technique (pdf, 270K) Surface Inhibition/Wash Slow Setting (pdf, 290K) Tearing/Rough Surfaces (pdf, 354K) Tight Fitting Crowns (pdf, 186K) Tray Seating (pdf, 304K) Voids (pdf, 436K)
Making Better Impressions – Troubleshooting Guide (pdf, 1.2MB)