Hi.I had this with my DD #1.She always had 3x2hr naps then went to bed at 6.30pm until she was about 25 months.Then it stopped completely.She would be so tired by about 4pm and try to go to sleep.
The only thing I could do was to try to get her to sleep in the mornings,about 11ish.As she was always (and still is) up at 6am this was a ideal.The first week were a problem,but for the first few days I would take her out in the car to make her sleep.She would never sleep in her bed or cot during the day from birth,so I couldn't do PU/PD,she would sleep on the sofa.
These naps lastest about 2 months,then that was it.she was a little girl who wanted to play.
I know it's hard when you have another lo.Maybe you can try the pu/pd again for a while.
Let me know how it goes.