Hello. 

Yesterday, I checked Molosser Dogs 's site and found out that the proble appears because of incorrectly registered license. It may happen if
1. the app was used on some old domain and wasn't reregistered when domain was changed.

2. the app is used on more then one domain, while one license allows to use the app on one domain.

In case of point №1 you need to go to My Sites (https://una.io/page/my-sites) -> Licenses block and check the Domain (Key from Key&Secret pair) which the license is associated with. If it's different than your current Key (can be checked in Studio -> Settings -> General -> "UNA account key" setting), you need to use 'Reset' to reset the association. Then you need to go to your Studio -> Apps Market -> Purchases. You'll be asked to login to your UNA account, if you aren't logged in yet. After that the paid app (in this case it's Articles) should appear there, it means that the license was reregistered. And after that the updates for the app should be available in Updates subpage.