Undecoded SACCH means the MS did not succeed in decoding the DL SACCH msg.
After the “HO Command from UTRAN” the MS should send HO ACCESS in UL to the GSM channel. Do you see those ? Right after that, you should expect a handshake in the layer 2 (SABM, UA)
Does the info in the HO COMMAND FROM UTRAN is correct ? Is the 2G frequency (ARFCN) provided in the msg the good one ?
If HOSR is 99%, then there is no reason that your MS is not able to succesfully go to the 2G cell during drive test. Can you trace your DT mobile call on your OMCR 3G and OMCR 2G ? Then you will have aclear picture of what’s going on.