DC-DC boost converters in parallel to run DC motor
When powering circuits from low voltages you have to be very aware of voltage drops that may occur.
The leads from the battery to the circuit should be of heavy gauge, if you use a battery holder it can drop significant voltage (any springs used to contact the battery may be made of steel that has much higher resistivity than copper, I had to add additional wires across the sping on one design I did to minimize the voltage drop).
It is also difficult to measure the current on the low voltage side - a typical DVM may drop up to 200mV on the current range and the meter leads can drop hundreds of millivolts so the combination can lose up to half a volt.
Single-phase boost converters can take much higher peak currents than their average - the peak current can cause the supply voltage to drop below the cutoff limit and prematurely restrict the output current. Good low ESR capacitors close to the boost converter can help. You may need several thousand microfarads of tantalum or polymer capacitors - aluminium electrolytics may not have low enough ESR to help.
Why are you restricting yourself to a single  AAA cell? The space taken up by the circuitry may end up as much as an additional cell or maybe use a Li-Ion cell. Its 3.6V terminal voltage will avoid these low voltage issues.