normally you won't get a good charge if you're using the tablet while it's plugged in. some might indicate they are charging but often it is just draining either way depending on the output volts of your charger. i always charge mine to 100% at the end of the day and the next morning take it off and use it. i usually get anywhere from 7-10 hours on a charge.
When you said it's off and plugged in, the indicator is just a charging indicator that often displays in the middle of the screen. if it's showing red, it often just means you're charging a very low battery, and it will turn green after it's at least 25% or more charged up. if you see a triangle with exclaimation point it means your battery is either faulty, your charger is bad, or charging is suspended due to being too hot or too cold.