A combination of using anyJustPressed and insert fixed things. Thanks!

I am checking for FlxG.keys.anyPressed([SPACE]) and setting the result to the _space variable.

Also, forgot to mention this earlier, but this game has a public GitHub repository, here. If anyone needs to look at full code, it's there.

Here's the gist of what I'm trying to do:

class PlayState extends FlxState
{
	private function generatePiece():FungalPiece
	{
		return new FungalPiece(FungalPieceColor.Red, FungalPieceType.Single, this, 40, 64);
	}
	
	public function nextPiece():Void
	{
		var _nextPiece = generatePiece();
		add(_nextPiece);
	}
}

package;

import flixel.FlxSprite;
import flixel.FlxG;
import flixel.math.FlxPoint;

class FungalPiece extends FlxSprite
{
	private var speed:Float = 200;
	private var _active:Bool = true;
	private var _playState:PlayState;
	
	public function new(color:FungalPieceColor, type:FungalPieceType, play:PlayState, ?X:Float=0, ?Y:Float=0, ?active:Bool=true)
    {
        super(X, Y);
        _active = active;
        _playState = play;
        // a bunch  of code here to initialize the image
    }

    private function movement():Void
	{
		if (_active)
		{
                    // move the piece code goes here
			if (_space) // if user presses the SPACE key
			{
				_active = false;
				_playState.nextPiece();
			}
		}
	}
}

When nextPiece() gets called, the current piece is locked, and a new piece is generated and is supposed to be added to the PlayState. This is causing the game to hang / freeze. There's got to be a "correct" way to do this; I just do not know what it is!