Can't stop Y-Axis motor

Hi all,

My printer is Prusa i3 with RAMPS 1.4 and four 4988. All functions are fine, I can print anything what I want.
Today, I try to use Repetier to stop motor, X and Z motors release but the Y-Axis didn't.
I can't move Y-Axis manually by my hand.

I try to find what is the issue, so I try those steps...
1. Exchange the X/Y pin on RAMPS 1.4 and then X-Axis lock. So it means the Y motor is fine.
2. Exchange all 4988, Y-Axis still lock which can't move by my hand. So it means four 4988 are fine.

I have no idea what the problem is? Is Arduino Mega board problem? Please help! thanks