Please follow these steps to refresh the license information on the computer. It can resolve some activation problems. 

1. Close all instances of DraftSight
2. Navigate to the  location: ~/Library/Preferences/Flexnet Publisher
3. In that location, locate a folder named 'Flexnet'.
4. Rename this folder to Flexnet_old.
5. Once done, restart DraftSight.
6. During the restart process, you will be prompted to enter a serial number.
7. Follow the steps to reactivate the license.