[Zope] Custom ZClasses absent from addmenu after upgrading

I am in the process of upgrading a site on NT from 2.3.3 on one machine to 2.5.1 on another. I copied across the data.fs, reinstalled a few products and everything seems to be working OK apart from the addmenu which stubbornly refuses to list a number of custom ZClasses. They list OK in the Control Panel, existing instances still work and I can copy/paste to create new ones. Any suggestions much appreciated. Peter