How long do you keep her awake for? At this age my dd could only really handle about 1 and half hours awake. If I try to keep her awake for longer than that she gets overtired and only sleeps 30 to 45 mins.
My DS has never napped very well (and he was going for entire days without sleep at 12 weeks, but was sleeping all night). Babies have to learn to go from one sleep cycle to the next without fully waking up (around the 30 minute mark) - I think this takes time to do. I think the advice you've got here is good, but I would try not to get too stressed about it. I know that is easier said than done, but all you can do is provide the right environment and lead up to sleep. My son is just a short napper - he sleeps now at nearly 12 months, for an hour and a half tops (and that is rare), but he tends to nap more frequently. IME, the daytime naps have just got better over time, in terms of a routine. I also read somewhere that months 0-3 babies focus on nighttime sleep and months 3-6, babies focus on daytime sleep. Not sure how much truth there is in that, but I do think 12 weeks is very young. I am sure things will improve.