Is it the right crimp connector for the cable area? (mm^2 or AWG) A connector for a larger area than is used can give a bad connection and/or slip off even if crimped with the right tool.
Is the tool of good quality? Some tools just don't work well, even for their intended use.
Many tools, particularly the better ones, are dependent on being used in the right direction - I recently made the mistake of putting the connector in back-to-front, and the cable slid right back out of the crimped connector.
Some tools have adjustment knobs, that need to be set to the right connector size, others have different slots for different sizes. Is it set/used for the correct connector?
Are the connectors good quality? Some connectors, particularly cheap ones, are the wrong size for their color code (if applicable), or not made to specifications.
That's all I can think of at the moment.
Uninsulated and heat shrink is good too, they can usually even be soldered.
