Michelle --
By what you say in your question, it sounds like you are not correctly
working with a project in Offline mode. Instead, it sounds like you are
saving your project as an .mpp file, which is a local file rather than an
offline file.
The Offline mode is meant for users who travel away from their network
connection, such as while on an airplane, but still need to work with their
enterprise projects. To save a project in Offline mode, open the enterprise
project while connected to Project Server, and then click File - Save
Offline. When you are traveling away from your network, do the following:
1. Launch Microsoft Project Professional
2. In the Project Server Accounts dialog, select your regular user account
(not My Computer)
3. Click the Work Offline button
While working in offline mode, click File - Open and you will see your
offline project listed in the Open from Project Server dialog. As you work
with the offline project, simply click Save to save the changes.
When you return to your corporate network, launch Microsoft Project and log
into Project Server normally. When you click File - Open, you will see your
offline project listed with all of the other projects. Open it and then
click File - Save Online to synch up the changes you made. Hope this helps.