The tunnel is up but traffic doesn't flow. Debug systematically:
Routing: Is traffic reaching the tunnel interface? Check your routing table.
NAT issues: Is NAT changing the source before the packet enters the tunnel? Exempt VPN traffic from NAT.
ACLs: Are access lists blocking the interesting traffic or the encapsulated ESP traffic?
MTU: Encrypted packets are larger. If MTU is too small, packets drop silently.