If the player has the ability to use Pal Park, the dongle method allows specific wild Pokémon to appear depending on the GBA game with an 8% chance (4% each if there are two different Pokémon that can appear this way due to the same game). If the player cannot use Pal Park, the dongle method has no effect.
The only thing that may prevent you from encountering Pineco is if you don't have access to Pal Park. You must have the National Dex to access Pal Park (and therefore be able to encounter Pokemon via the Dual Slot method), which is obtained by seeing every Pokemon in the Sinnoh Pokedex.
If you do have access to Pal Park, then the 8% encounter rate and bad RNG may be the cause. Or your game may not register the Pokemon Emerald Game Pak in the GBA slot. :P
Hope I Helped!