Sorununuz hala devam ediyorsa eger,

private bool mouseIsDown = false;
private Point firstPoint;
private void Form1_MouseDown(object sender, MouseEventArgs e){ firstPoint = e.Location; mouseIsDown = true;}

private void Form1_MouseUp(object sender, MouseEventArgs e){ mouseIsDown = false;}

private void Form1_MouseMove(object sender, MouseEventArgs e)
{
if (mouseIsDown) {
int xDiff = firstPoint.X - e.Location.X;
int yDiff = firstPoint.Y - e.Location.Y;
int x = this.Location.X - xDiff;
int y = this.Location.Y - yDiff;
this.Location = new Point(x, y); }
}
İşinizi görecektir.