Here's a link for info on headset standards, scroll to the bottom for a nice chart showing the pertinent measurements for the different standards:
http://www.parktool.com/repair/readhowto.asp?id=68

It sounds like you may be using JIS cups on a frame intended for an ISO headset. But if it turns out you're using the correct headset for your frame and you still don't get the proper interference fit, Park Tool recommends using Loctite #RC609 to secure it-