A few reasons invites could be seen as expired.
1) User error: The invites were actually expired through time or limited uses. (Have them manually revoke the invite, then generate a brand new one)
2) You're in 100 servers already and have reached the server limit.
3) You are banned from the server, or you share an IP with someone who was banned. (Do you frequently use a public access point, college WiFi, etc?)
The solution to the last problem would be to contact a staff on the forums.