After the dropController has finished a sequence it sends an end of sequence message back to the app. “<FTS>” (Finished The Sequence). It appears that the app is not getting this message.
In all my tests it works fine; Nano, Mini, UNO, Duemilanove, Mega,
When I get chance I will reintroduce the debugging version of the Windows app. This will allow you to see what is happening inside the app and sketch.