1.

When the model has been substantially updated, the clash details remain in the Clash Manager database (not the model database) even though the item(s) they refer to have been removed. In these circumstances, the particular clash details continue to be reported until all Clash Sets containing the detail have been rerun.
These clash details, including their comments, can be removed from the Clash Manager database by use of the 'Delete all Unreferenced Groups and Elements, including Comments' button on the Clash Manager Options Form - Advanced Tab.
This process removes historical data and consequently the original comments will have been lost, if the clash re-appears.
Refer to Advanced Tasks.