I saw sometimes similar problem. C/I is between 30 and 35 dB, but MS stay on coding scheme MCS6 in downlink. Probably MCS6 is defined as starting coding scheme. At first check if you have Link Adaptation – true. You could try behavior of data downlink transfer during busy hour and during midnight (probably no other user will used EDGE).
If the problem is only during day, than the problem could be on ABIS (check PM counters), if there is enough space. 4 radio timeslot for MCS9 for Siemens technology need 20 ABIS subslots. Then check counters for PCU. One PCU could serve only some limit of PDCH (ABIS and radio channels)