I've been playing a Cyber Dragon deck for a few days now and today suddenly Cyber Dragon Nova's effect that states:

"Once per turn: You can detach 1 Xyz Material from this card, then target 1 "Cyber Dragon" in your Graveyard; Special Summon that target."

Isn't working. I have a Cyber Dragon already in my graveyard before this card is summoned but the activate button wont appear for this effect.

Can anyone help with this issue please?
There are many reasons that this may not be working.

Please post a replay stating when you wanted to use the effect but could not, and on which server and game version this occurred.