My baby doesn’t sleep. What am I doing wrong?

My baby is almost 6 months. Very active, spends 2+ hours on the playmat, gets lots of walks, cuddles and attention from both me and her dad.

About a week ago, I’ve gotten strict with the nap schedule:
- Wake up at 7.30
- First nap from 9.00 to 10.00
- Second nap from 12.30 to 13.30
- Third nap from 16.00 to 16.45
- Bedtime at 19.30

She is still EBF and I nurse her to sleep every night and for every nap. She sleeps in her own crib.

However, every other night, she wakes up around 23.00-00.00 and stays up for 2 or 3 hours. I nurse, rock, bounce, let her play… nothing works. On these same nights, she then wakes up at 5, 5.30, 6 and until it’s the time to wake up at 7.30.

Other nights, she sleeps perfectly through the night (only waking up to feed at around 1 and 5 in the morning).

Please, what am I doing wrong? The sleep deprivation is so real at this point.