Few reason this could happen. Voltage Dropping or Phase Converter is overloaded.
1. You could be having a voltage drop due to, weak service, too small or too long of wire. Lets check this first.
Go to your single phase circuit breaker panel, place a voltmeter on the breaker that is supplying the phase
converter, if the voltage is not dropping below 208 volts while try to start the phase converter then your service
is sufficient, next go to your phase converter panel, place your voltmeter on L1 and L2, if the voltage drop
much lower then your circuit breaker panel while starting the phase converter then your wire is too long or
small, if it is not dropping then your wire size is sufficient, finally go to your equipment and check your lines
for voltage drop there while trying to start you equipment, if it drop only at this location then your three phase
wire suppling the phase converter is undersized. If none of these apply for voltage dropping then go to #2.
2. Place your voltmeter on L1 and L3 inside the phase converter panel, try starting your machine, if the volts drop
below 165, while the single phase line remain above 208, then the phase converter is too small or your equipment
could have issue to address.