Problem with winform redrawing itself

I am building a Space Invaders game. I have got a simple algorithm that removes an invader when it gets shot and adds a score.

The problem is in this bunch of statments:

   if (removeInvaderBool)
            {
                  // removeInvader is an Invader object which was hit, and to be removed.
                Invaders.Score += removeInvader.addedScore;  //Invader class has got a property with a specific score (e.g. score 50). Score is a static field in the Invaders class.
                invaders.Remove(removeInvader);//invaders is a List<Invaders>
            }

Now 80% of the time, the score is being upgraded and the invader is being removed. The other 20% of the time, the score goes up, and the invader isn't being removed...

Why is that..?

My game is on winforms, the Form1_Paint event handler draws all the graphics every 33 ms, and the game timer runs every 10 ms. (I use 2 timers, instead of threads.)

Why is it that 20% of the time the invader is not being removed while the score is being updated??

here is a bigger extract of my code as requested:

       foreach (var invaderItem in hitInvaders)
            {
                // if the area of hte alien contains the shot remove the shot.
                foreach (var player in playerShots)
                {
                    if (isWeapon)
                    {
                        //Me: Will remove players shot.
                        if (invaderItem.Area.IntersectsWith(player.Area))
                        {

                            removeShot = true;
                            removePlayerShot = player;
                            removeInvaderBool = true;
                        }

                    }
                    else
                    {
                        //Me: Will remove players shot.
                        if (invaderItem.Area.Contains(player.Location))
                        {
                            removeShot = true;
                            removePlayerShot = player;
                            removeInvaderBool = true;
                        }
                    }
                }
                // remove the alien that contains 开发者_Python百科the shot

                removeInvader = invaderItem;

            }
            if (removeShot)
            {
                Invaders.Score += removeInvader.addedScore;  
                playerShots.Remove(removePlayerShot);
            }
            if (removeInvaderBool)
            {

                invaders.Remove(removeInvader);
            }
        }

Debug your program. List<>.Remove() returns a bool. Make use of that:

            Invaders.Score += removeInvader.addedScore;
            bool ok = invaders.Remove(removeInvader);
            System.Diagnostics.Debug.Assert(ok);

The debugger will break when the invader is not in the list. Consider that you may not have implemented its Equals() method correctly.