We're been going through this exact thing the last 2 months with DS!! He is 2 year 4 months, and used to nap really well, around 11am, and We'd wake him by 1pm at the latest. He'd fall asleep for the nap quickly too. But about 8 weeks ago he started taking longer and longer to go to sleep at night, one night it was 2 hours! He wasn't upset, just rolling around in his cot singing and playing with his soother. We checked in every 15 min.
So we started limiting the nap shorter and shorter, but it didn't make any difference, he'd still the a really long time to fall asleep. So now we're stopping him napping at all and he falls asleep max 10 min.
However, we're now stuck in this limbo where he gets really grumpy late afternoon, but if I let him sleep, he then takes double that length of time to fall asleep at night, but he sometimes wakes up really early too. Today was 5am, so I let him nap for 30 min, and he took over an hour to get to sleep after we put him to bed at 7pm. We're just taking each day as it comes.
So, sorry I don't have any answers, just sharing what we've been through!