Try to make the collision polygon as round as possible. If this doesn't help, you have two options:
Pretty sure I tried experimenting with that but I'll do it again to check.
1. use Physics with circle collision mask.
I've tried this a while ago. I think it improved the way things bounced but I couldn't see any way to exclude collisions with objects of the same type so I couldn't stop the circles from colliding with each other making it unsuitable for this game.
2. use raycasting feature of LOS behavior - cast a ray forward to get the correct reflection angle, and then bounce the ball at that angle.
That sounds like it could be worth a try. I haven't used raycasting before so I'll look in to it.
Thanks for the ideas!