Please be sure you are copying and pasting just the serial number itself -- sometimes when you copy/paste text, an extra space or line break finds its way in at the end of the serial number.
Check that the version of the app you have currently downloaded matches the app license you originally purchased. Typically, if you bought version "2.x" of an app, but the current version is "3.x" you'll either need to buy the new version from MacUpdate or find the old 2.x version of the app to register using the old serial number.
If you’re still having trouble, contact the app developer directly. You can easily find the developers website by navigating to the apps info page on MacUpdate.com and clicking the developer name in the upper right hand side. Most developers will have a Support form or email address directly on their site.
Didn't find what you're looking for? Feel free to contact us directly.