Movie clip movement help

1

I have four buttons that move the clip up, down, right and left. Here is the script I’m using.

rightBtn.onPress = function() {
        b1 = true;
}
rightBtn.onRelease = function() {
        b1 = false;
}
leftBtn.onPress = function() {
        b2 = true;
}
leftBtn.onRelease = function() {
        b2 = false;
}
upBtn.onPress = function() {
        b3 = true;
}
upBtn.onRelease = function() {
        b3 = false;
}
downBtn.onPress = function() {
        b4 = true;
}
downBtn.onRelease = function() {
        b4 = false;
}

small.onEnterFrame = function() {
        this._x += (b1 - b2) * 4;
        this._y += (b4 - b3) * 4;
}

I’m getting some strange results though. Basically if I want to move the clip left it wont work until I click on the right movement button first. Same thing with up and down. I have no idea what’s causing this.