Digital Certificate installation hassles

I have purchased a set of client digital certificates from a major supplier to enable a set of users to encrypt emails on their Outlook clients. The supplier uses a browser-based download. In all cases the download proceeds and then states "OK". On some PCs the user can then send digitally signed emails just fine, and then receive Encrypted emails (and read them); but on other PCs, Outlook refuses to sign the emails and also can't de-crypt; on these latter PCs I additionally can't then export the complete digital certificate from IE - only the public part is available, the private part has disappeared or can't be accessed Any ideas? - I cannot see any differences between the PCs that work and those that don't.