Can't digitally sign and encrypt email any longer.

I used to sign and encrypt my emails digitally and then send them to people. I had two email certificates from Comodo, and it's been a while they're expired. so I renewed my certificates and deleted the old ones from my keychain, then downloaded and added the new ones. The trouble is I no longer get the digitally sign the message and encrypt the message button when I compose a new email. What could be the problem? how can I fix this?