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If, for any reasons, the syncing has failed and did not recover automatically, you will need to check the status and reset it. For that, use the Entity Sync Status feature in Webtools. If it's still in running state, put it back to not started status using "Reset Run Status" (it depends of translation), in order for
the job to pick things again. Note that these jobs should run only on the client(s) only and you should not run multi-instances of them. So you only have to care about this issue from the client(s) side.
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