What equipment are you using?
For Ericsson Network:
1.Check the definition of neighbors it should be mutual for both neighbors.
2.Check the measurement frequency (MF). The Bcch of neigbor must be define at the MF of serving cell.
3.Check the external cell definition. The neighboring site which is homed at the other BSC should be define at the serving BSC.
4.Check the synchronization of all cells. Cells need to be synchronized for algorithm to take place.(better ask support from core engineers on this matter)