TFS will not accept changes I've made to a Jav

问题:

I am using TFS's Team Explorer to manage Visual Studio projects.

Recently, I've created a new Java project (not in Visual Studio) which I manually added to TFS using the Source Control Control explorer in Visual Studio.

After I added the Java project to TFS, I made some changes and bug fixes. Then, I went into Visual Studio and opened the Source Control Explorer to check in my changes, but TFS thinks that no changes were made.

It seems that I needed to check out the project before making changes. I guess I erroneously expected TFS to track that automatically, but okay.

So, using the Source Control Explorer in TFS, I checked out the project, and then I tried to check in pending changes. When I tried to check in, I got the following message:

All of the changes were either unmodified files or locks. 
The changes have been undone by the server.

Is there a way to convince the server that indeed my project has changed? How can I check in the changes that I have made?

Thank you for any help.