WAG straight arm back extension rolls

[twistingmia]

I can do a bent and straight arm back extension roll on trampoline, I can do a bent arm on floor, but today I started the straight arm and I can't do it on the ground.

I guess I am definitely strong enough, but I just don't get it.

Is it possible to get a perfect straight arm back extension roll in 3 months if I train it 5 times a week for 15 minutes each day?

Thanks in advance