Two possible reasons that the S_QOS_DATA table is not being populated by the data_engine probe:
1. The hub_bulk_size key value in the <setup> section of the data_engine.cfg file is set to a value that is larger than the number of rows in the S_QOS_DATA table. To resolve this, change the hub_bulk_size value to a value less than the number of rows in the S_QOS_DATA table from the data_engine probe's Raw Configure GUI, apply the change and restart the probe. Wait a couple of monitoring intervals to allow probes to publish new QoS messages and the S_QOS_SHAPSHOT table should start being populated again.
See: data_engine Best Practices -> Hub Bulk Size
2. If you have a smaller system that is monitoring less than 20 QoS metrics, change the value for the thread_count_insert value to zero from the data_engine probe's Raw Configure GUI, apply the change and restart the probe. Again, wait a couple of monitoring intervals to allow probes to publish new QoS messages and the S_QOS_SNAPSHOT table shoul start being populated again.
See: data_engine Troubleshooting->S_QOS_SNAPSHOT Database Table Missing Updates
NOTE: Both of these key values are found in the <setup> secttion of the data_engine.cfg file