You may need to check a few different things, given that your guest OSes can either use their own time sync processes or VMware Tools.
If it's via VMware Tools, you would need to check a couple of things:
1. Does every ESXi host have NTP correctly configured and running?
2. Does every VM have tools.syncTime = "1" in it's VMX file?
I think PowerCLI would be able to help you there.
If your guests are syncing directly via their own OS processes, that mix of OSes makes it harder to find a "one size fits all" solution.