Get some decent logging going so we can see what's actually going on. Array ios working great; UI is not being redrawn based on the variables (like in Android).

lib/main.dart

@@ -1,5 +1,22 @@
 import 'package:flutter/material.dart';
 
+//import 'package:logger/logger.dart';
+//final logger = Logger();
+
+import 'dart:io';
+
+class MyLogger {
+  void d(String output, {String method = ""}) {
+    stdout.write(DateTime.now());
+    if (method.isNotEmpty) {
+      stdout.write(" - $method");
+    }
+    print(": $output");
+  }
+}
+
+final logger = MyLogger();
+
 void main() {
   runApp(const MainApp());
 }
@@ -123,6 +140,9 @@ class _BoardSpotState extends State<BoardSpot> {
         setState(() {
           if (widget.text == "Reset") {
             gameReset();
+          }
+          if (widget.spot >= 12) {
+            null;
           } else {
             gameTurn(widget.spot, BOARD[widget.spot]);
           }
@@ -213,9 +233,14 @@ marbleRemove (int spot, int marbles) {
 */
 
 void gameTurn(int spot, int marbles) {
+  logger.d(method: "gameTurn", "spot = $spot, marbles = $marbles");
+  if ((boolTopPlayerTurn && spot <= 5) || (!boolTopPlayerTurn && spot >= 6)) {
+    logger.d("Not this player's turn, try again.");
+    return;
+  }
   marbleRemove(spot, marbles);
   while (marbles > 0) {
-    if (spot == 6 && !boolTopPlayerTurn) {
+    if (spot == 5 && !boolTopPlayerTurn) {
       marbleReward(boolTopPlayerTurn, 1);
       marbles--;
     } else if (spot == 11 && boolTopPlayerTurn) {
@@ -223,6 +248,10 @@ void gameTurn(int spot, int marbles) {
       marbles--;
     }
     if (marbles == 0) {
+      logger.d(
+        method: "gameTurn",
+        "Landed in player hole, go again! board = $BOARD, score = $SCORE",
+      );
       return;
     }
 
@@ -243,6 +272,7 @@ void gameTurn(int spot, int marbles) {
   }
 
   boolTopPlayerTurn = !boolTopPlayerTurn;
+  logger.d(method: "gameTurn", "Finished, board = $BOARD, score = $SCORE");
 }
 
 List<int> BOARD = [];
@@ -250,32 +280,40 @@ List<int> SCORE = [];
 bool boolTopPlayerTurn = false;
 
 void gameReset() {
+  logger.d(method: "gameReset", "Started");
   BOARD = [];
   for (int i = 0; i < 12; i++) {
     BOARD.add(4);
   }
   SCORE = [0, 0];
+  logger.d(method: "gameReset", "Finished");
 }
 
 void marbleAdd(int spot) {
+  logger.d(method: "marbleAdd[spot=$spot]", "Started");
   BOARD[spot]++;
+  logger.d(method: "marbleAdd[spot=$spot]", "Finished");
 }
 
 void marbleReward(bool boolTopPlayer, int marbles) {
+  logger.d(method: "marbleReward[$boolTopPlayer, $marbles]", "Started");
   if (boolTopPlayer) {
     SCORE[0] += marbles;
   } else {
     SCORE[1] += marbles;
   }
+  logger.d(method: "marbleReward[$boolTopPlayer, $marbles]", "Finished");
 }
 
 void marbleRemove(int spot, int marbles) {
+  logger.d(method: "marbleRemove", "Started");
   // This should always be taking it down to 0, but doing the math anyways
   // just t make sure nothing wonky is going on, like 4 being removed then
   // 5 being moved across the board or something. Which would likely be unrelated
   // code elsewhere, but oh well.
 
   BOARD[spot] -= marbles;
+  logger.d(method: "marbleRemove", "Finished");
 }
 
 class ButtonReset extends StatelessWidget {