The issue might be due to any one of the following.
1. Wrong settings of HORXLVDLI / HORXLVDLO. Pls specify these settings alsong with GSM / DCS tilts.
2. Hardware Issue at DCS. be it the TRX or the combiner. Check level and Quality stats for TRXs.
3. HOM on neighbour level but that should have same issue with both calls on both GSM / DCS.
4. Tilting Issue with the DCS if you are using a separate tilt configuration or separate antenna.
5. TX power Issue at the TRX check PWRRED setting of DCS TRXs.
Plus there could be a few other reasons.
Hope this is of use..