what is your current radio throughput per TBF ?
what was and what is your value of MAX PDCH ?
MAX PDCH is just a guideline for your MFS. If there is some CS trafic in your cell, the MFS cannot allocate MAX PDCH, but only a computed value called “MAX SPDCH LIMIT”.
If you really want to increase your PDCH versus your TCH, you should also increase your MAX PDCH HIGH LOAD and increase your HIGH_TRAFFIC_LOAD_GPRS.
But as i said, now the algorithm of PDCH allocation is quite “automatic” and you can’t do much with parameters. Rather, you could try to increase your half rate occupancy, or add some TRX, or share the CS load with neighbour cells.