Baby Sleep: Toddler Resisting Naps
A common explanation for why naps are difficult to accomplish is that the bedtime is too late. The current bedtime of 6:30 PM might be fine if she was napping every day, but she isn't. When the bedtime is too late, she wakes up in the morning too tired. Then she might go down too early for a nap that is not in synch with her biological nap rhythm, or she might be kept up too late for this nap and develop a second wind. Either way, the end result is the same: a brief nap, no nap or a barely restorative nap.
Experiment with a bedtime of 5:30 PM for several nights and see if she falls asleep at this earlier time. If so, then you know that 6:30 PM was too late for your toddler's nap. I would expect her to have more net night sleep even if she wakes up in the morning slightly earlier. More net night sleep means that she will be able to more easily fall asleep and stay asleep for a single midday nap.
When she is napping better, consider a slightly later bedtime—say 6 PM or perhaps 6:30 PM. Always watch her behavior between 4 and 5 PM (which will be proportionate to how long and when she took her nap) to determine whether you will put her down earlier or later.