I noticed this was happening on one of my SQL instances.

In order to fix the problem I followed these steps:

1) right click Data Collection and select Disable Data Collection (this disables the SQL Agent jobs)

2) stop the SQL Server Agent service

3) opened task manager and killed all DCEXEC.exe processes (I think these were hung)

4) start the SQL Server Agent service

5) right click Data Collection and select Enable Data Collection

All pending cache files were processed and new ones were being re-created again after doing this.