2xAA + 5V Boost Converter: Unexpectedly Low Current Output
Boost converters are rated by input current, so a 600mA boost converter can't make 600mA at 5V with a 3V input 

With alkaline cells you should design for 1V per cell unless you want to waste money replacing the AAs when they still have 80% charge,
For 600mA at 5V from 2xAA input you should use boost converter rated at more than  1.5A. eg:  XL6009.