To manually specify a member switch as the master, power on that switch first, and then wait 20 seconds before powering on the other switches.
The master election timeout period is 20 seconds. The length of time each member switch spends in startup may differ significantly. Therefore, when the stack is powered on or restarts, not all member switches participate in the master election. Member switches that start more than 20 seconds later than the first switch to complete startup cannot be elected the master switch.