First you need to investigate what are the AMR codecs used by the MS, and see for which codec there are call drops.
And also the HO split : after AMR activation, new HO causes are enabled (intracell mostly) –> how much do they occur, are they replacing some of the HO quality that occured before activation ?
About parameter values, I can’t help. If you’re using default values, then you need to spend time analyzing the QoS before tuning them.