The Unity manual states that an object with Physical Material Bouncyness of 1 will bounce without any loss of energy.
I have a Ball with a bouncyness of 1 set to ignore gravity, yet it gradually slows down after bumping into other rigidbodies.
Why?
This is a bit of a dumb question… but is there any drag/angular drag on the rigidbody?
Otherwise, and I’m just speculating, could it be friction?
I’m presuming you’ve got the ball in some kind of totally enclosed container and it is bouncing within it. Maybe due to friction the ball loses some of its tangent velocity with each bounce? … Although if you are bouncing it exactly back and forth between two walls then I don’t know.
If this is still a problem for you maybe post some of the other values and the pattern of the bounces.
Check the ball’s Physics Material: is Bounce Combine set to “Maximum”? If not, do so: your ball won’t loss energy. 
Here’s a brief explanation of what happens.
The Friction Combine and Bounce Combine captions define what happens when two different materials collide. Let’s suppose you have an object with Bounce 0.4 and one object with Bounce 1, and they are going to collide.
- Maximum: both objects will react to the maximum Bounce value, so 1.
- Minimum: both objects will react to the minimum Bounce value, so 0.4.
- Combine: both objects will react to an average of the Bounce value, so 0.7.
- Multiply: both objects will react to the multiplied value of their Material, so 0.4*1=0.4. I never tried this, so I don’t know how it will react precisely!
Hope this helps.
Set frictions to 0. 