When your MPLS neighbors are not forming the neighborship, you may have these possible issues:

• MPLS IP not enabled on interface
• protocol missmatch (TDP, LDP globaly or at interface level)
• higher loopback ID is taken as a router ID which is not advertised via IGP
• missmatch authentication if configured
• filtering udp port 646 for LDP
• etc

Commands to verify

• show mpls interfaces
• show mpls ldp neigh
• show run int {xxx}
• show run | sec mpls
• trace XXX