Think it's fairly normal - you've just got to make sure the charger is on a firm and reasonably level surface and that you firmly insert the batteries into the charger. Just make sure the light for the battery is lit up in either green or yellow. If not, then just give it a firmer push!!
I've found if the charger hub is on an uneven or soft surface (like carpet) then the port doors don't always lie flat and the battery doesn't register in the hub, you can sometimes knock one off charge when you push another in and tilt the hub slightly.
Not the best of designs - looks great as a design, but falls short in the real world!