Ok well, the difference in amps wouldn't really cause any issues and to be honest the increase wouldn't probably either with a 10amp supply. So I've got a few more questions.

What type of PSU were you using and what voltage.? ie: Toridal or linear PSU, regulated or unregulated, and how many motors pulling from them 3 or 4.

How are the motors wired, parallel, or series and what Drives are you using.? If you have the motor datasheet post it.

What Control software and did you re-tune for the new motors.?

One of the main problems could be the motor spec, if it's a cheaper motor then the inductance will most likely be very high meaning you'll require a much higher voltage to get the same speed as the much smaller and lower inductance Nema 17.
If this is the case then I suspect your problem is more lack of volts than Amps and your speed/torque curve is much lower. This is the problem with upgrading, if the system isn't balanced then you can actually make the machine worse, not better.

I'm happy to help and give advice on the forum, or if you prefer I'm also happy to speak on the phone, it's often much easier and quicker to help fix problems so just drop me a PM and I'll send you my numbers.