OSPF area type mismatch
When two OSPF routers attempt to become neighbors, there are several "sanity checks" that take place that must be passed in order to achieve an adjacency. One of these is the area type.
Two OSPF routers that are attempting to become neighbors that exist in the same OSPF area, must have the same network type configured for that area. For example:
R1(config)#router ospf 1 R1(config-router)#area 1 stub R2(config)#router ospf 1 R2(config-router)#area 1 stub
Otherwise, neighbor adjacencies will not form.