Not everything you say is true.
IF the classes are in the jars, and IF you have set the classpath correctly, THEN you would not get this message.
I suspect you have a build path confused with a classpath (which is mostly associated with runtime, where build path is associated with compile time). Without knowing more details of your runtime environment, it's hard to be more specific.
It always puzzles me how someone with a problem they cannot solve asserts so definitely that all these things are set up "correctly": how do you know? Isn't the problem some evidence to the contrary, at least enough for a small doubt?