If you haven't already, you might want to measure the OD of your existing headset's frame cups (e.g., the parts that insert into the head tube).

If your existing headset is ISO/Italian/French/Campagnolo Pro (the latter 3 types are now obsolete), they should measure 30.2mm OD. JIS/Raleigh headsets should measure 30.0mm OD.

JIS/Raleigh headsets in an ISO frame's head tube likely won't yield a proper interference fit. I'd guess that's not the problem here - but confirming the OD of the headset is correct for your frame would eliminate one possibility.