Buck-Boost converter - Boost mode not working
The data sheet states that the minimum start-up voltage is 1.9 volts so it can only be a couple of things that are causing this problem: -


Your input voltage is dropping below 1.9 volts under the heavy load conditions on a cold restart
The line/wire impedance to your converter from the power supply has too much resistance/inductance
Maybe it's a PCB layout problem (any breadboard layout will be problematic for this type of design)
Your measurement of the input voltage is incorrect.
Bad chip.


So go thru this list and try and figure out what is the problem. Double check the circuit you have shown - I see discrepancies with the data sheet - pin 10 is FB not pin 11. Pin 9 is ground in the data sheet yet you have pin 9 as VIN A. Most strange.