I have implemented Laser printers using USB printer servers connected to a SCO server. Sometimes print jobs get stuck on the queue and I havent found a way of getting the printer back to work without canceling the jobs and rebooting the system.

Why jobs get stuck?
How do I get this solved without canceling and rebooting?

Try /usr/lib/lpshut to stop the print service , and then /usr/lib/lpsched to restart it, rather than rebooting the system, after canceling the job that still shows as on the printer in lpstat -t.
Are you using PCL print commands?
Do all the print files have CRLF line termination?
Do you have <esc>&k2G in the escape sequence (turns LF into CRLF)?

Yes, I we are using PCL print commands, however, i am not sure about CRLF. Today the jobs got stuck again, i stopped and started print services before and after deleting the jobs but it did not do anything, I had to cancel all the jobs and restart the server.

I'm not sure about these two:
Do all the print files have CRLF line termination?
Do you have <esc>&k2G in the escape sequence (turns LF into CRLF)?

How do I get that information?

Are you using the HPLaserJet printer model?
Are you sending raw and text files to the printer?
To find the contents of the print files add a 'cp' statement to the driver file in /usr/spool/lp/admins/lp/interfaces to save a copy somewhere, then examine the file using 'vi' or whatever.
Also see what escape sequences the printer model might add to the print stream though echo statements.

Yes, I'm using HPLaserJet and printing text files only.

Its not likely that the problem is caused by the content. Can you try the following. Connect the printer to a networked workstation running MS Windows and then set up the printer on SCO as a network printer, using either VisionFS or Samba. Then even if the problem still occurs you will not have to reboot the server, and if it doesn't recur you can probably assume the problem is somewhere in the usb driver or the firmware.