there's a chance it could be the plug itself. Unbeknown to a lot of people, a glowing plug does not necessarily mean that the plug is good. The theory goes like this. For the glow plug to keep glowing after you have intially started it, it requires a catalytic chemical reaction between the platinum alloy element and the methanol in the fuel. What happens in a catalytic chemical reaction? The catalyst is used up. This is why a plug can happily glow brightly yet no matter how hard you try it wont start an engine because the necessary catalyst has been exhausted. So basically dont always think a glowing plug is ok. Try another.
Of course it goes without saying this is after you have eliminated other factors like the guys above have mentioned. It could be infact your charger. Not sure what charger you guys have but mine glow plug igniter takes about 8+hrs to charge.