Custom planetary gearbox doesn't have enough torque
 

I have 3D printed a custom designed two stage planetary gearbox with a total reduction ratio of 16:1. To test whether this truly is the reduction ratio, I tried rotating the input shaft 16 times using a NEMA 17 stepper motor and the output shaft does indeed do 1 complete rotation. My question is, now, is it safe to assume that, roughly, the torque will be amplified by a factor of 16 as well?

I ask this question because the output torque is far too less than what I expect. Typically, the NEMA 17 provides around 0.3 N⋅m. So, the stall torque of the planetary gearbox should be around 4 N⋅m (a bit less than 16x accounting for losses). However, the maximum torque it can provide is around 0.42 N⋅m, which is an entire magnitude less than what I expect. I tested this by hanging a water bottle a certain distance away from the pivot.

So is it safe to assume that the problem is with the NEMA 17 just providing insufficient torque (since, as I mentioned, the gearbox does rotate 16 times for every one rotation of the input, meaning it should amplify the torque sufficiently)? If this is the case, how can I resolve it? I'm using a4988 drivers to drive this thing. The reference voltage is around 0.5 V and the chip gets really hot when it drives the motor. Is it just a faulty driver issue? Or am I missing something?