Special move doesnt do any damage

Im doing the event challanges for the extreme units (androids, nappa, vegeta, raditz and hero ginyu). Im stuck in the android 20 challange where I cannot perform the special move with android 20. Wheb i click the blue card it just comes up with animations but doesnt do any damage and doesnt count.
I have tried SoulBoosting the Special Move art but it still doesnt work. Pls help.