The problem is the source path. Particularly under tomcat. Under tomcat you place the dependency jar files under the web-inf/lib directory. As far as eclipse is concerned these are not jar files.
Because they are not jar files you can not open them and put a break point. You can do this if you move the jar files a level up and define them as a dependency to the webapp. If you do that more than one webapp might use the same jar files and thereby breaking the static variables.
The work around I have used is to define a new java project that just holds these jar files as build dependencies of that project. This project is constructed just as a holder of the jar files. Don't forget to define the jar files as build dependencies. This allows them to be shown up as expandable jar files in the file viewer allowing you to open the source file and put a break point.
In the remote debug configuration allow this project as one of the source paths.