Stealthman gives great advice. If it fits with an erection but not without, did you measure hard or soft. Since we don’t live with constant erections (if so, see your doctor and BRAG) measuring with a bones is going to give a wrong size. It sounds like it could either be a size issue or penis shape (stealth can make a custom fit). I know we all hate to hear it could be a size issue. Not your dick but the Stealth ?.
If you have questions you can feel free to message me.