Z axis has 2 motor not smooth

[wirat]

Some time left motor higt 10mm right motor higt 20mm not equal when run quickly
How can i do ?
Some time left not move right move
Used a4988 chip
Thanks
Regards

[Mark]

Looks like your motor is out of step. you can adjust the current of your motor to see if it works, generally it arranged from 0.8-1.5A.
the way to adjust is as the pic shows: