Bingo! that's the problem. your arm setup looks much more like a 3D airplane! you are getting about half the force available of the servo at the elevon with that long servo arm. Before buying
a stronger servo, try to use a servo arm with half of this lenght and tell us what did you found....

K.Greene, the Futaba radios can achive 140% of servo travel. I am actually using 110% in my roo
with the short arm... (plenty of throw on my elevon on my plane)

Best regards, Enrique