Hi, a plate is the best way to fix to parts of a broken bone so that it joints in proper alignment. Looking at your arm from outside you would have skin and the surgical scar. This covers a layer of muscle, tendons and nerves and blood vessels - all or which would have been affected somewhat by the injury
and have healed by scar formation. Right inside is your plate holding on to bone beneath. This has also scar tissue of healing which obviously is not visible to the naked eye. Scar tissue and its embedded components can cause pain off and on. Rarely there can be issues with the plate and its hold on the bone. If the bone has not healed well enough - plates can loosen or break. My suggestion - get a review by your doctor , he may get new x-rays. If all is OK anatomically physical therapy
may help to relieve your pai.