Hey everyone!
I have spent about 4-6 hours looking at this code, and different code examples with no success.
Basically, I'm trying to do a "Tetris" animation on the NeoPixel 8x8 Matrix using the GFX, NeoMatrix, and NeoPixel libraries. To cut down on memory usage on the Arduino, I am trying to create as few functions as possible. Due to that fact, I am trying to use one animation function to make a red 4x4 box move to the bottom of the matrix, then calling that same function, I make a blue box drop on the side of the red box. Unfortunately, when I call the function again with a different color value, it doesn't work. It just hangs there. I narrowed it down to the code just "floating" at the end of the for() function (by float, I mean that it just waits there, and doesn't do anything i.e. restart). I have been pulling my hair out for the past few hours, trying to figure out why this code won't work! Please have a look at my example code! 
Simply put, I'm trying to make the red box move to the bottom, STOP, then have a blue box come down next to the red box, but while using the same function I used for the red box.
Thanks for your time, and help!
Have a good one! 
#include <Adafruit_GFX.h>
#include <Adafruit_NeoMatrix.h>
#include <Adafruit_NeoPixel.h>
#define PIN 6
Adafruit_NeoMatrix matrix = Adafruit_NeoMatrix(8, 8, 6, NEO_MATRIX_TOP + NEO_MATRIX_LEFT + NEO_MATRIX_ROWS + NEO_MATRIX_PROGRESSIVE, NEO_GRB + NEO_KHZ800);
Adafruit_NeoPixel strip = Adafruit_NeoPixel(64, PIN, NEO_GRB + NEO_KHZ800);
int delayval = 250; // delay for half a second
uint32_t red = matrix.Color(255, 0, 0);
uint32_t orange = matrix.Color(255, 137, 0);
uint32_t yellow = matrix.Color(255, 255, 0);
uint32_t green = matrix.Color(0, 255, 0);
uint32_t blue = matrix.Color(0, 0, 255);
uint32_t purple = matrix.Color(204, 0, 204);
uint32_t black = matrix.Color(0, 0, 0);
uint32_t white = matrix.Color(255, 255, 255);
void setup() {
matrix.begin();
matrix.setBrightness(16);
matrix.show();
}
void loop() {
fillRect(0, 0, 4, 4, red);
fillRect(0, 0, 4, 4, blue);
matrix.show();
}
void fillRect(uint16_t x, uint16_t y0, uint16_t w, uint16_t h, uint16_t color) {
int y = -3;//x-axis starting point.
for (;; y++) {
matrix.fillScreen(black); //This goes before to "wipe" the following rectangle pixels.
matrix.fillRect(x, y, w, h, color);
y = constrain (y, -3, 3);
matrix.show();
delay(delayval);
if (y >= 3) {
y = 3;
}
}
}