Hi OP, are you having 2 or 3 sleeps per day? At 6 months DS was suddenly able to stay awake for another half hour and went down to 2 sleeps. As pp suggested also try a 15 min winding down before a nap by reducing the stimulation and doing something quiet and calming - DS used to love to have a cuddle and look out of the window at the trees before afternoon naps. Some deep breathing from me also encouraged (and still does) to calm down and get ready for sleep.
I believe what I actually thought was over tiredness was me putting her to bed too early, now I stretch her out and she has been (fingers crossed) going down a lot easier, it was like you said, suddenly she could stay up an extra half hr, so 2.5 hr up in the morning and usually nearly 3 in the arvo so 2 sleeps a day with a big usually 2.5 hr in the morning and 45 in the arvo, but she's been down to 2 for about a month now.
TPS Health Physiotherapy and Pilates has three clinics located at Morningside, Redlands and Lutwyche. We offer pre and post natal services as well as physiotherapy and Pilates. All clinics offer child minding services so bubs are always welcome!