This usually happens because Signal Spy was downloaded and installed with a different Google account than with which the original purchase was made. So, make sure you download Signal Spy with the same Google account as the original purchase was made with and your Pro status should return.
Sometimes you just need to completely close down Signal Spy and reboot your phone to see your Pro status return.
As always, use the Contact Us form if you have further questions or have trouble getting back your Pro status. When you do contact us, send us your original order number from your purchase with your zip code, so that we can check for your original purchase information. (We are unable to check for your purchase information with anything except this order number as we do not have access to personal information attached to your payment information.)