First up, i'd take a drop of both products, and run them together in one run down the hplc, to be sure you're seeing two species.
Its possible you're seeing an N-acylurea of arginine (formed by a slow rearrangement of the active O-acylurea intermediate), can you be sure the product contains any of the original amine? And as pointed out by agrobert, you need MS data ideally.