Agree with Masao13… sounds like you need a size smaller in girth.
How are you for length? If the sheath is too long it can also contribute to slippage. As a passive extender the innerwear needs to be sized 1/2″ less or so than full shaft extension. This amount of room will allow the penis to yield to the forces of retraction that occur when sitting as the pelvis shifts back with forward rotation of the hips. This is the acid test for the correct size with respect to slippage.
Another consideration is the shape of the shaft. Is it relatively straight or contoured? If needed we can custom cut the sheath to fit accordingly. It’s not uncommon for Stealth users to require this option.
Another good point Masao brings up and is probably the biggest challenge when dealing with slippage. Is there a less pronounced coronal ridge so that the silicone ring cannot properly seat itself to grip and maintain positive extension?
What size are you wearing now if you don’t mind me asking… if you prefer we could discuss these details in private (sorry that we don’t yet have the PM feature)