You have two VPCs that you've peered. You created a route for VPC A to get to an instance in VPC. You are unable to ping the instance. You have double checked your security groups and NACLs.
Why might this be?

A. You forgot to add a return route.
B. ICMP is not supported over peering connections.
C. You have to enable Source/Destination check in the VPCs.
D. You have to configure the peering connection to allow two way traffic.