If you’ve caught a bunch already that don’t have the nature + ability combo you want, then you’re probably locked out of having it.
Cute charm normally works by forcing the wild Pokémon to have 1 of 25 PID values, 1 for each nature. The glitch activates when 1 or more of those values would be shiny with your save file’s TID/SID combination, up to the maximum amount of 8. As such, when using the Cute Charm glitch, only 8 natures will be shiny. These 8 natures will always be shiny, and the other 17 natures will never be shiny.
PID also determines the Pokémon’s ability in Gen 4, so a Pokémon with a specific nature generated by the Cute Charm glitch will always have the same ability, as well.
Source 1, source 2