There is no point in increasing SDCCH TSs when you have SD drops, you should add SD TSs when you have SD blocking.
In your case, with 1/1/1 if you have reconfigured with 4 SDs that leaves only 3 TCHs, which is a poor planning.
SD drops might be due to different reasons. I guess the first thing is to reconfigure those SDs back to TCs so that you won’t loose traffic.
Further let me know your BSS vendor, then I can tell you some troubleshooting mechanisms.