When this happens, it usually means the authentication has expired.
Make sure the time on your phone and workstation are within 1 minute of each other. If they are out of sync, you may not not see the option to approve or deny the authentication.
Another reason this could happen is if you have multiple devices registered to your Trusona user account. When you are logging into your workstation, each device will receive an authentication request. If you approve or deny the request on a device, you will not have the option to approve or deny the authentication on another device as the authentication has now expired.