Hi G Kelu,
First of all, I am not (yet?) optimiser but BSS reporter so there is room for mistake or lack of precision in my answer. As I could judge, the main problems (from most common to most rare) are:
coverage holes (could also be indoor), neighbour links (you should have enough but not too many), SDCCH congestion (near LAC borders generally), TCH congestion (need capacity extention or activation of some features), interference and/or quality, bad azimuth/tilt.
Other ideas are welcome too. I am also curious :oP