Why must I always use Rebuild Project and make project and not just compile my file ?

I use to be able just to compile the file I want to run (ie: a simple junit test), and all its dependants files (and only them) were compiled.
Now I often need to make a Rebuild project or a make project (what is the difference ?), in order my test can compile, otherwise I get very strange messages telling me that some symboles cannot be resolved (while when I go on the file everything seems just fine)... clearly the .class are out of synchronize and I have to do the rebuild... but before idea was detecting it by itself... I want this back, please !!!

And you know, when you have a big, big project, this is very frustrating...