Driving a 50W LED from 10 rechargable batteries (XL6009 converter @25% efficinecy)
Ah. The usual "I did not consider conservation of energy" confusion:

The power flowing into your converter must be the same as coming out of it. Power, for DC current, is simply voltage · current.

If you want to step up voltage by a factor of \$\frac{37}{10\cdot 1.2}\approx3\$, then you'll inversely reduce the current supplied – so, to produce 0.5 A at 36V, your step up converter would have to draw 1.5 A. And that is probably far beyond its reach.

Also, your batteries only supplying 4A in shortcut is not a good sign. NiMh usually has much higher short circuit current. But in that case, the voltage drop at 1.5 A might already be large enough to inhibit operation.

EDIT: you picked a step-up converter that doesn't even have a maximum current or maximum transferred power specification. It's hence the most likely reason that you're trying to make your step-up converter source more current than it possibly can. Use a step-up converter that is either rated for the power you want to draw or the current at the voltage you want to draw.