The straight legs should actually make the skill easier not reduce your power. If you have straight legs your front foot will be further over your hands making it far easier to transfer your weight over.
When doing a back walkover start with your leg up at horizontal, imagine there is a string attaching your elevated leg to your hands, as your hands go back, then your foot has to go with it.
Are you keeping the leg you are standing on very straight and locks tight. If not this can cause compensations that make it hard to keep your front leg straight too.