This is always a comunicatioin issue(obviously) but since you don't state which port your hooked up to weather is serial or USB, if its USB make sure you have the correct windows driver installed, make sure you LXI software has the exact plotter model chosen(this will not be the case if your plotter is newer than your software program), if its serial connection make sure that your port setting on your computer match those of your machine(plotter)like the baud rate and stop bits and flow control, the flow control usually the cause of this.