Jumping in here, as I'm in the EXACT same boat! My ds will be 6 mos in a few days, and I have a crazy boy who'll be 3 in a couple of months. I'm experiencing the same nap issues, and am working with A times as well.
One thing I'm finding is that the morning A time is the shortest of the day. I'd been putting her down after 90 min, then upped it to 2 hours, and that seemed like too much (still only 45min nap), so I've split the difference and am shooting for 1:45. Sometimes it works, sometimes she still wakes up after 45. She too always wakes happy. I've tried to extend her naps, but it's never worked, and I also find it too difficult with the boy running around and wanting to see what's going on! I remember too with my son that his first A time of the day was always significantly shorter than the others, it tended to get longer as the day went on.
I think it can be harder with us bf moms, as you need to increase frequency of feeds to increase your supply (especially if she's having a growth spurt, and I believe there's a good one at 6 months?), and that can throw off the routine a bit. I would just be sure to make each feed a full on, so she doesn't get used to snacking.
Just my two cents, hope it helps. If I have any breakthroughs I'll let you know what worked for me. Good luck!
Andrea