Some Pokemon are un-tradable on the GTS, you can request them and throw them on there but you can't initiate a trade for them. Mostly event-only fair like Genesect, Mew, Shaymin, etc.
A local, or wi-fi person to person trade for said pokemon works perfectly though.
Is that Diancie hacked in? If it doesn't have a fateful encounter flag, proper event OT name and ID and a lot of other stuff, the servers WILL recognize it as not legit and won't allow to trade it online.
Did you update the game? A 1.3 update rolled out recently and it's required for online to function properly. If you did and it doesn't work, then dunno, maybe it's untradeable.
Sometimes when a Pokemon is distributed over Mystery Gift, they restrict trading it for a while. Even then, event Pokemon are permanently restricted over GTS.
There's something odd going on with the new Diancie's. I tried to use her in a passerby battle, but it blocked me thinking she was a hacked pokemon. She wasn't. I got her from a Mystery Gift code from GameStop. Same applied to my attempts to trade it. A friend wanted me to EV train her, but it wouldn't let him trade it. I tested mine and it wouldn't let me trade mine either.
My best bet is they haven't added this particular Diancie to the online check system they have, so she's still considered a hacked pokemon since it doesn't recognize her trainer information. Most mystery gift pokemon have specific trainer information and they may not have added the gamestop event in yet.
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