Hi, guys, here is the solution to the problem of process id com.sample.evaluation, and may be for any other process .
I just made small change to the application and saved it without doing any material change. The eclipse recompiled and build the application again and now the process instance diagram is visible.
The problem is only related to display of graph by jbpm plugin.
In summary, doing a dummy change to source code and getting eclipse to recompile and rebuild the process, brings back the graphical display of the process .