My understanding is that unit co-ordinator's have to open these manually - as in, they don't just open up once all the other classes are full.
Email your unit coordinator and let them know the available options are full and you are yet to register - they may open the other class or simply put you in with an existing class. The latter will generally happen if you are one of a handful only who are at present unregistered as it isn't worthwhile/practical for them to have a class with only a few students.