SPRINGFIELD — Successful postconviction petitions can be difficult to come by.Such requests are often initiated by defendants themselves and must survive a three-stage process before a court decides if some relief is warranted.If denied, a defendant typically forfeits any other constitutional claims that weren’t argued. Only a small exception in the law allows him or her another chance — if he or she can show cause for not raising those other arguments the first time and show that prejudice resulted.Such …