I don’t know anything about siemens, so I cannot help you.
If the same thing was happening in ALU (a system that I pretend to know), i would check all parameters related to the sharing of PDCH and TCH.
The number of ts allocated as PDCH is not a static value, but dynamic. I would check thresholds and margins that dynamically define how many PDCH are set when there is a high load of TCH.
Some indicators show how many PDCH are allocated at the busy hour.
Anyway, since you are already in TCH high load (as soon as you have TCH blocking means that there is 0 TCH available –> therefore the number of PDCH is restricted !) then it proves that you wouldn’t be able to anything more..
if you add more PDCH, then the TCH blocking will sky-rocket !