It is good know that you should never relay on calling
garbage collector exprlictily. So you must know that
methods that java provides are requests and not
demands. The virtual machine will do its best to do
what you ask, but there is no guarantee that it will
comply.
Anyway, you can also try caling GC through Runtime
class. For example:

Runtime r = Runtime.getRuntime();
r.gc(); // this is an alternate to System.gc()