Not sure if this is really about the exchange of gifts or whether you want to know if you have fallen out with your niece about something of which you are unaware.
If it's the former why is it important that you exchange gifts? Surely you can go on giving her gifts without the expectation of receiving one yourself?
If it's the latter, then honesty and candidness are always better in my opinion, especially if you are both near in age and have usually gotton on with eachother. Try giving her a call and arranging to meet up for a drink, etc. If she seems disinclined or makes an excuse that doesn't ring true just ask her if you have done something to upset her and take it from there. It might just be something as simple as the fact that she is skint in which case respect her circumstances and stop worrying. Good luck and let us know how you get on! x