This issue has happened because in the value MaxJobsCount* in the Configuration.xml has reached value of 51.
To increase the number of jobs in the processing queue, please follow the steps below:

1. Stop the ABBYY FineReader Server Processing Station and ABBYY FineReader Server Server Manager services.
2. Browse to %PROGRAMDATA%\ABBYY FineReader Server 14\Configuration.xml and backup this file
3. Open the Configuration.xml file and find the value MaxJobsCount, change the value to the number you need.
4. Save the file.
5. Start the services stopped in p. 1.

*The MaxJobsCount attribute sets the number of jobs allowed in the processing queue. We recommend specifying a value that is twice the number of Processing Stations in the group whose jobs are queued. The default value of the attribute is 50.

This setting covers most FineReader Server configurations, which have up to 25 Processing Stations. If your configuration uses more than 25 Processing Stations, increase the number of jobs allowed in the queue accordingly to prevent Processing Stations from standing idle.
Location in Configuration.xml: <OCRServer> → <QueueManager>