Cannot activate Vorachnid's ability

[huynguyenquang]

I played MD and could not activate Vorachnid's ability. I had Avenger of the Fallen and Gravebone on the field. Could not activate Vorachnid's ability for the first few turns. After that he works just fine.
The game was Huy Nguyen Quang vs Tapakah

[bobrossw]

was his attack the same as opponent ally cost? It needs to be greater.

[huynguyenquang]

Oh sorry I think they were the same. Tks bobrossw.