Transfer between Pokemon SoulSilver and Pokemon Emerald does not work

Hello, I hope you can help me. I'm currently completing the Pokedex in Pokemon SoulSilver and for the Hoenn Pokemon I need the GBA Pokemon games, including Pokemon Emerald.
I'll try to describe my problem to you now.
To play Pokemon SoulSilver I use my Nintendo DS Lite, which also has a GBA games slot, which makes it possible to play GBA games on my Nintendo DS. This works. I was able to play Pokemon Fire Red and Pokemon Emerald on my Nintendo DS Lite without any problems and I was also able to transfer 6 Pokemon from Pokemon Fire Red to my SoulSilver save game without any problems. Unfortunately I can't do this with Pokemon Emerald. The option to transfer Pokemon from Emerald is not shown in the Pokemon SoulSilver main menu.
Could it be because I have to play through Emerald first, because I played through Pokemon Fire Red and it worked then.
Could you please explain it to me?
Thanks in advance