Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
The funny thing about this story is that there's not really much of a plot, so it doesn't easily fall into a classic plot analysis. As you can probably tell, there is no primary conflict, no climax, and no resolution—those terms kind of fall apart with Woolf. Still, conflict is suggested in passing throughout the story. For instance, the snail's struggle to get around the leaf, or William's patient care taking of the old man. There are definitely no resolutions here, though—Woolf leaves the larger story lines underlying these minor conflicts to our imagination.
The fact that the story doesn't stick to a traditional plot arc tells us that it's a pretty experimental piece of writing. Who would have thought that a story in which a snail and some people move through a garden could be so engaging? Ultimately, it might be better to think of "Kew Gardens" not so much as a story in the traditional sense, but as a vignette or snapshot of a particular moment in time.