[Al Skierkiewicz]

Dave,
Usually failures in ratcheting crimpers occur when the terminal is positioned incorrectly in the tool. I know it sounds trivial but these tools are designed to crimp in particular fashion. The tool sold by MCM for instance requires that the terminal be placed in the tool from only one side. Since these tools are commonly designed for right handed installers, the insulated terminal should be placed in the tool so that the open end is is facing to the left hand. The terminal should be placed in the jaws of the tool so that the insulation is centered in the jaws and that the terminal is facing up (parallel to the jaw face). The correctly stripped wire is then placed into the terminal. If the wire is stripped properly, the end of the wire should be visible protruding from the terminal opposite where the wire was inserted. Then operate the tool and you should have a good crimp.
When the wire is not stripped properly or inserted fully, the crimping action will push the wire out the back of the terminal much like squeezing a banana. If the terminal is inserted such that the seam inside the terminal is not facing the jaw, then the terminal will not close properly on wire during the crimping action and will pull out. On many of the tools I have used, the jaws are marked for the color terminal for which the tool is designed. In general, the colored markings are the side of the tool in which to place the open end of the terminal.
Remember yellow terminals are for #10-#12 wire, blue for #14-#16, and red for #18-#22. It is also important to note that many of the ratcheting crimpers also have adjustable crimp force. If your tool has a small indexed wheel on the side with + and - marked on the tool, simply remove the locking device (usually a small screw), rotate toward the next + mark and reinstall the locking device.
Finally, the Thomas and Betts crimper many of us have received in a rookie KOP over the years is designed for uninsulated terminals when using the dimple part of the tool.