If you feel sync isn’t working correctly, these are the first things to check:
In DEVONthink on the Mac, see what is reported in Window > Log. Then check the Troubleshooting > Sync Issues section of the built-in help.
In DEVONthink To Go see if there is a triangle on the sync icon in the bottom toolbar of the database screen. If so, tap it to see what is reported. Then check the ? > Help > Sync > Troubleshooting section of the built-in help.
If you need further assistance than you can glean from the help or the downloadable manual, you will at least have some information to pass along to the support team.