When you are working on a workshared project file it is always a good idea to keep an eye on the Active Workset. Whatever the Active Workset is set to, this is the Workset all of your new modelling will be created under.
You can find the Active Workset in 3 locations.
In the Collaborate Ribbon, In the Worksets dialogue box, or the most convenient which is located at the bottom of your modelling area.
The Active Workset can change without you realising, resulting in your work being located on the incorrect Workset. This is because different users change the Current Active Workset in their local file. When they synchronise it will make their Active Workset as the default Active Workset next time you open the Central File.
The Moral of this story is to keep an eye on the Active Workset, when you next open (or create) your Local file.