Could you just elaborate, what was the earlier system which was swapped to Nokia ?
For SDCCH drop rate, Nokia takes sum of all failures occuring on RF/Abis/Aif/LAPD/BTS/Usr/BCSU and some of them during handovers. So it is quite high. I hope you have this in mind.
There is other reason too, but this is due to MSC settings and the drop rate would be high on the PLMN boundary sites.
Which one relates to your SDCCH drop ?