Doing two modest pokemon at once, the first 4 were fairly easy - I got each one on both pokemon within the first few attempts at each grass spot. Serebii lists that there are 7 possible locations - 4 worked, but the other 3 simply aren't. Do I simply keep trying? I tried all the remaining 3 many times with no such luck.
Go to the summary of your pokemon, on the menu that shows their nature. On the top screen, just below where it says where you met the pokemon and what its characteristic are it will say Shiny Leaf then next to it show how many you have, they glitter. For me it shows one then an empty space then 3 more together.
I got it to work. On Bulbapedia, it directly tells you which route the leaf you are missing is on (on the stat page, it literally lists each of the 5 leaves, and whichever ones are missing you can look up with exact route it is on)