My DD is almost 23 months and until v recently would take a 2+ hour nap after lunch no problem.
Now she is starting to resist this some days. More details below - does this look like the beginning of the end of naptime? 
This week:
Sunday - resisted nap but crashed at 3.30 for 45 mins
Monday - resisted nap but eventually nodded off after I went in to settle her and slept for just over an hour
Tuesday - took a long time to get to sleep but self settled then slept for 1.5 hours
Weds - slept for 2.5 hours no problem!
Today - resisted nap and is going strong, will try to keep her going til bedtime now
Naptime routine is nappy change, short story, sleeping bag then self settle in cot. Usually she chats for up to ten mins then nods off.
When she resists a nap she doesn't get upset, she just keeps on chatting until I go and get her. I leave her for at least half hour.
Bedtimes have also been a struggle this summer - we put her down at 7 but she won't usually go to sleep unless DP or I sits in with her and this can take an hour or more. I think the light and hot weather are at least partly to blame for this. Wake up time is very reasonable - between 6.30 and 7 usually.
If she won't nap in the cot, walking in the buggy doesn't work anymore - she finds it stimulating and I just get ' Where we going Mummy? Where we going Mummy?'
She isn't teething afaik and is happy when awake, no illness.
Should I just keep offering a nap each day after lunch and if she doesn't sleep at least she has had some quiet time in her cot? What approach did you take?