I used to have this problem, but found a solution over at AFF by serfty who also posts here.

Basically you need to set so your itineraries go into "Unfiled items". To do this you need to go to the top right of the home screen and click on your name. A drop down menu will appear with various items. Click on Settings. Then click on "About You" on the left hand menu. Go to the bottom of the page. Under Travel Preferences uncheck the box marked "Put new plans into trips".

One way to solve the problem is to resend your flight data. Once it has arrived, then delete the old itinerary. Then go to unfiled items and click on make a copy (under the options menu at the right). Then adjust the itineraries by deleting the necessary flights (eg. delete inbound flight and so keep outbound flight in one and delete outbound flight and keep inbound flight in the other.) Then click on "move" from the options menu and create your trip name or move to an existing trip.

If you have more flights that you want to break up into separate trips then simply make more copies of the original and edit as necessary.

I hope this helps.