This would be a behavior if for whatever reasons storage to FileCabinet by Import job fails. The reason of the failure shoudl be shown on documents in the tray - just use table view instead of thumbnails. Most common reasons:
Disk Capacity Limit reached (and no automatic disk advancement is configured)
or if "Select list only" is used on fields bein indexed by the Import Job - the error "Value must be from Select list"
if metdata file used in Import - wrong name of the field in metadata files