Lo started dropping naps, so I switched his awake times to the 2 3 4 method. After waking for the day, up for 2 hrs then 1st nap, then up for 3 hrs, then nap. Up for 4 hrs then bed. Seems to be working for the last 3 weeks. Hope this helps.
I would try this, you might not make it 4 hours in the afternoon, ours was more of a 2-3-3. I think it's the evening nap that is the issue. Eliminate that first, and work on the morning nap, make sure you are doing it at the right time. It would be highly unlikely that a 9 month old was ready for 1 nap. Most kids go to 1 nap around 15 months.