I asked topguest and got the following answer: "The reason you haven't received the reward is because the venue you chose when checking in with Twitter was the name of the city and not the actual name of the hotel unfortunately".

They referred me to "How to Choose Your Location" on http://support.twitter.com/articles/122236

Why they want that is understandable, they want to have the property as location name. But it's kinda wrong to accept the first sign-in without proper venue name but not the others. Getting the venue name with my twitter client is kinda annoying and I also can't write the same message on twitter twice. So I switched to foursquare.