I upgraded my Motorola ES 400 firmware and lost the PTT key mapping.

Zello Paid support

We believe that was caused by a change in the firmware. To make the side button available for Zello please perform the following steps:

1) Go into key remap utility and select “Library – reset All” to establish a common baseline

2) Next map the PTT button to hard keyboard – PTT (short press) [basically remap PTT back to itself]