and what is her usual wake up time? is it somewhere between 7-8am every day ??? Or later???
As this will affect the advice given.
assuming she wakes between 7-8am, then on no nap days I'd say she wants to be going to bed even earlier than 8pm. Probably 11hrs max from when she woke in the morning & you might find she settles quicker & sleeps longer. if I do that with my DS he will generally pull a 13hr night, whereas if I keep him up til his usual BT he will take ages to settle & wake early the next day.
If she typically wakes well before 7/7.30am, then on nap days I'd say 8pm BT was too late & to try even earlier, but if she is waking post 7.30am most days then it is also possible she is undertired. Her awake time in the morning before her nap is quite short at 4-5hrs max, and so at the moment its quite possible she needs to be awake for 6-7 hours after her nap, hence why she is fighting BT, IYSWIM?
I found with my DS when he got to this age that he needed to be awake for longer in the morning (6hrs) before he took his nap & this had a positive impact on how well he settled at bedtime. So we went from this at 18 months
Up: 7am
Nap: 12.30-2.30pm
BT: 7.30pm
to this at ~20 months
Up: 7am
Nap: 1.00-3.00pm
BT: 8pm
Pushing the nap later so he was awake nearer to 6hrs before his nap meant he I found with my DS when he got to this age that he needed to be awake for longer in the morning before he took his nap & this had a huge impact on how well he settled at bedtime.
HTH.x