A Narcissists charming behaviour lasts as long as they get what they want from you i.e. that you provide evidence to confirm their view of themselves as special. In any relationship where people spend a lot of time together it is normal for the parties involved to notice the others bad habits and behaviours. To some degree it is normal behaviour for people to point these bad habits and behaviours out to the other person in order to try and improve the relationship. Narcissists do not react kindly to this, they see it as an attack on their perfection. As a result of this their behaviour starts to change. The lovely person you know will start to disappear as the damaged part of them emerges, the things they do to impress you will become fewer and farther between, you will start to feel less and less special to them as they start to treat you badly and show their frustrations (overtly or covertly) at your inability to meet their needs more frequently. This is known as your "Devaluation".