I think 12mA pins are not driving properly. How to go about?

The main effect when driving 24 versus 12 mA to a LVCMOS load is reducing the edge rise- and fall-times, most likely introducing some overshoot if no additional termination is present, and generally reducing the effective signal delay.

Personally I try to use high drive strength only if it's absolutely needed, e.g. for clock signals that need fast edges. In any case, it's better to check the waveforms at the load with a suitable active oscilloscope probe.

If a particular signal seems to have problems with 12 mA drive strength, this is mainly indicating a timing problem. Increasing the drive strength is only one of many ways to handle it.