It suddenly happened that the SDCCH drop rate increased ? Did you change frequencies or the antenna design ?
When something like this happens (and I didn’t change anything on the site), I usually suspect a hardware problem.
Try resetting the Abis link, the TRXs, etc…
Then you can try to change the frequencies of the cell, and eventually go drive-test around this area.
As said ealier, check the Mean Holding Time (MHT). What is it’s vallue ? What about the TCH assignment success rate ? The Call drop rate ? The TCH MHT ?
Good luck Silvia, let us know how it goes…