Oi! va voy!
If it was that simple to just melt an already tempered bar more people would be making chocolate. When you get a shipment of chocolate (whether you buy from any of the producers i.e. Guittard, you still have to temper the chocolate. Bars typically weigh 10 lbs.
You need to melt the bar to around 120F. Then you need to add a few lumps of the tempered chocolate from the remaining bar or new bar. At that point turn the temp down to 82F. At this point you need to mix the batch until it reaches around 82F. Remove the lumps and turn up the temp to around 90F. Keep mixing until you reach 90F. At that point you can begin using the chocolate. HOWEVER, you need to maintain that temperature throughout the process otherwise it will go out of temper.
Now,there is a lot of trial and error in making chocolate. You need to practice until you get it right.
Best of luck,