I also witness such a co-site HO failure on our network some moonths ago.
In order to solve the issue, I’ll advice you to check the followings:
1. Check the BCCH frequencies of the cells
2. Check the BA lists of the cells (Confirm if the BCCH of the target cell is present in the BA lists of the serving and vice-versa)
3. Check the HO Relation definition btween the 2 cells (Mainly the CS parameter as suggested by Da Architect)
4. Run a drive test in the area to check interferrence levels
5. Make relevant corrections in the BA list;
Moreover, If you are used to the OSS, please initiate a Consistency Job in the CNA. This will surely help you to clean up you network BA List wise.