Sometimes with older ProCurve switches, you just have to manually link them...
Sometimes I find that deleting and re-adding a device helps: everything looks like it is fine, with the correct SNMP, SSH, etc settings, but it just doesn't discover the link, or device completely. Make sure you are using templates for SNMP, telnet, SSH, etc. You can have multiple templates for each if you need to, but it avoids the occasional typos, especially if you have messy community strings, IDs or passwords.