To sum up your post:
Cell 1, TRE 1 = OK
Cell 1, TRE 2 = SDCCH DROP
Cell 1, TRE 3 = SDCCH DROP
Cell 1, TRE 4 = SDCCH DROP
Cell 2, OK
Cell 3, OK
1) on cell 1, all your TREs are carrying a SDCCH timeslot ? That is surprising ! If there is no SDCCH ts defined on a TRX, you can’t observe SDCCH drop on this TRX.
2) Are those standard “static” SDCCH ts or dynamic SDCCH ts ?
3) What about the tch mean duration on each TRX of cell 1 ? Do you see traffic on those TRX 2, 3 and 4 ? TCH DROP RATE is OK ?
What you can try to do :
1. perform drive-tests, perform call setups, and check for crossed sectors. Lock some TRX in order to force call setups on all different TRX.
2. unlock all TRX and put all the sdcch timeslots on the m-BCCH TRX (and follow up QOS)
And that’s it.. i hope it’s gonna help you.