S.P.
My daughter is only 8 months old but she certainly has a mind of her own and is only going to nap when she wants to nap. I have also noticed that she naps much more easily (and for longer periods of time) with my mom and her nanny. Eventually she naps with me when she is ready (and only when she is ready), or not at all. All I can make of it is that she's happy to have time with mommy and doesn't want to waist a minute of it sleeping! I definitely agree that its tiring, but maybe if you make your time with her extra special with fun activities it will make her sleepy and it doesn't have to be so routine.
Good luck!