Was not the rule set to the following?
If empty: Write fields
If not empty: Keep old values (never overwrite. Except for the Name field and one other I don't remember)
This could be the reason why the OP have some troubles.
I think the reason for this was......
People might name their series or movies with small variations. And this can lead to trouble later on. For instance automatic background images, who depend on the names. If there is no exact name hit, the closes one will be used, and the name will be overwritten. IF this is a mistake, and the wrong movie or series is selected, you can correct this by manually selecting the right series and movie. If you select the series, a unique code will be saved, and this should never get overwritten again.
The artwork for each episode is downloaded as screen captures. This is a standard used by the automatic meta data scraper. If you want to change this you'll have to convince JRiver of making some customizations to the system
This is all done for automation in mind, and there can occur some bad hits. This is however less work for those that want an automated system vs those that need everything to be 100% on the first hit. At least in theory