Try finding the app in the store and hit the buy/install button... It should do one of 2 things...
A) recognize you already purchased the app and let you install the "paid" version again (without having to pay) if the app still reacts like you haven't purchased it, contract the developer,
B) show a confirmation that you want to buy the app... If this is the case, for some reason the MS app store thinks you didn't previously buy it. You might check with MS rather than the developer as this would be more of an issue on their end.