Marc,
First, you should enable the logging system:
Go to menu
Window, then
Preferences, then
Convertigo Tab, then
Studio and enable DEBUG level in ContextManager, Context and Engine:
Then activate the Eclipse console and select the Engine output.
This logging level should help you understanding which Screen classes are detected and which transactions are started.
You can also write traces in the console.
Just add a new statement into your transaction and use the Log function.
This should help you finding your problem...