Just agreeing with Zoe, we had this too. I could never get the pm A right to get a longer nap. Tbh it worked a lot better for us to keep pushing that first A for a longer nap, then capping the afternoon nap. I think it works better when lo's have a habit of waking early to keep lengthening the first A slowly as it gives them more incentive to go back to sleep when they wake early in the morning.
I still think that BT could come a bit earlier again, maybe latest 6.30 if the naps have gone well.