Ok, it is good you said they are in the same layer. Check also they are in the same BCCH band.
Check the cell is defined as a neighbour in the OMCR. Is this a cell that belongs to the same MSC ? same BSC ?
Check that the traffic in the neighbour cell is not too high (high load in neighbour cell will slow down the better cell HO, or even inhibit it)
Check that HO_MARGIN(serv, neigh) = 5dB and A_PBGT_HO(serv) < 14 (normally, it should be equal to 8 or 10) Check that you didn't do a HO before, so you're not experiencing a PING PONG hysteresis. Ok, i think that covers pretty much evereything... Check also the QoS stats (INCOMING HO REQUESTS and TCH TRAFFIC ERLANG) in the neighbour cell. Verify that incoming HO are possible, and especially the better cell HO. Check per adjacency that HO coming from your serving cell are successful. Good luck,