.Net apps are not release memory

Question

We have one windows service application deployed in 64 bit machine, the memory used by the apps not get cleared after the execution. All the database objects are disposed and set nothing to all object , but still the memory clean up is not happening. Is anybody is having any idea why the GC is not cleaning the memory  used by the application. Initially this was running in 32 bit server and the performance was good , after installing it in 64 bit server with x64 as build option the performance is degraded.