It is possible, that some other program is stealing the shortcut.
1. Install latest SSMSBoost (3.7)
2. Install latest available service pack for SSMS
4. Check if you can assign to some other shortcut.
If still not working - please send us a message to support@ e-mail with About dialog screenshots (SSMS and SSMSBoost), so we will see the DLL versions, + commands dump
Please also mention, if you use other add-ins