1. What it the diameter of the drive pulley for the belt? Better yet what is your steps per inch setting.
Microstepping loses torque. Speed loses torque. Bigger motors lose torque faster with RPM.
2. What speeds are you trying to run at?
3 What kind of bit and depth of cut are you using?
Some drivers has "step morphing" (goes to full step above a set RPM) because microstepping becomes ineffective above about 100 RPM and eats up torque . Since you are limited by the driver for the total current it will allow if part of that is burned pulling against the rotation to smooth it out will not apply all of the torue to the load. In steppers, voltage = RPM and Amps = torque. Losing steps means your required torque to move the load and all of the forces of cutting exceeds the torque the motor can provide at that RPM and the motor "stalls" (loses steps)
There is a fine balance between getting max torque at the feedrates you are cutting and getting the upper speeds you need . One would think that just buying bigger motors helps but if the gearing is wrong it can actually hinder. You have more torque in low end but you run out of torque quicker at higher RPMs