How long have you been coding? I was writing RPG III for 18 years and only wrote sporadically in RPG ILE until recently, self taught from the manuals. I managed to pass the RPG Developer Certification from there.
You may just want to see where your knowledge is insufficient. I’m sorry that I don’t remember the link, but from IBM’s site, you should be able to locate a link that allows you to purchase and take a practice exam that will give you a pass/fail percentage by knowledge category.