1. No access to the files.
Check system permissions and make sure that on this computer the agent has read-write access to the files and all parenting folders. In job's details check which folders is set up for this agent.
2. Time difference between agents is more than 600 seconds.
Agents may be located in different timezones so they rely on UTC timestamps - this must be the same on all agents. Check UTC time on all agents and compare results:
OS X/Linux in Terminal:
Windows in Command Prompt:
for /f %x in ('wmic path win32_utctime get /format:list ^| findstr "="') do set %x
Run the command with minimal time lag to get more accurate results for comparing.
3. Files are currently being in use.
Close all apps that might be keeping the file and preventing other apps from accessing it.
4. This is a Read-Only share
If a non-synced agent has Read-Only permissions in the job, syncing will stop if a file gets updated on it. Since this is a read-only share, the updated file will not be uploaded. To fix this problem, make sure that in the Profile assigned to this agent's group the "Overwrite any changed files" option is set to YES.
5. This is a low-priority job.
If there's a job with higher priority and its being synced, this one will be waiting for its turn. See here for more details on job priority.
If neither of the tips helps, please collect logs from agents and Management Console, and send them to support.