Hi Chang,
ESD is a very complex issue, and there may be tons of things that can go wrong, and it may very well be a layout issue. From the schematic, the points I can say are:
1. Your gate to the Diff pair NMOS is directly connected to the PAD, the gate might get damaged easily if the ESD diodes don't have low enough resistance path to shunt the ESD current through to the main clamp. Is this I/O very far from the main clamp?
2. If the other transistor (whose drain is connected to nwell and then to pad) is getting damaged then maybe your Nwell resistor is not high enough.
3. Check your bus routing and calculate the total resistance seen by the ESD path that is failing. If its high then some node may be rising in voltage high enough to damage a device.