So I bought a universal laptop charger since mine got fried. Before I bought it, I asked the guy at Best Buy if it would charge since my old charger has 20V, 6.75A and this one has 19V, 4.75A. He said it would, it'll just be slower but it should keep it on no problem. So I went ahead and bought it, afterwards, I proceeded to charge my laptop for approximately 3 1/2 hours, turned it on and it didn't charge at all. Any reason why?