I have tried, and my PR application got rejected a couple of times. Should I file an appeal letter for rejection, or should I wait and reapply after six months?

You should appeal the rejection only when there are some significant changes in your profile, such as promotion in job, social contribution efforts, and change in the family nucleus. If there have been no improvements in your profile since your first rejection, you should work and upgrade your profile. You should also hire an immigration agency to ensure there are no errors in your application and that the submission is flawless.