LindaSur,
You have to understand that internal fixation is only there to hold the bones in close proximity, until they heal. The hardware is not designed to take the place of the bone, or to stand up to a lot of repetitive stress.
It is taught in orthopedics that it is a race when hardware is placed: what will happen first, the bone heals or the fixation fails?
So, if your humerus had a nonunion, then the hardware will fail. It is sort of like a paper clip that is bent over and over, will eventually break. Same is true with the internal fixation. It will either snap or its screws will pull out of the bone.
So, what has to be determined is why is your bone not uniting?
Usually, at this point, another attempt at fixation would be tried (be it another plate or possibly an IM (intrameduallary) nail. Also, bone graft (preferably autologous) should be place to help jump start the healing. External support with a co-apt (also called a clamshell) brace should also be instituted.
Make sure you are eating a well balanced diet, with extra protein, calcium, vitamin D, and magnesium. Do not smoke, which is probably the single most detrimental thing there is to bone healing.
Good luck. Hope your bone heals this time around.