If after five years of treatment for depression my lack of concentration persists over and over again. Could it be adhd?

Lack of concentratio. Further evaluation to confirm/rule out the cause can warrant further interventions.
Yes,no,maybe. Hyou could have co-existing adhd, however unresolved depression is more likely the ptoblem.Are you seen a therapist, taking meds, exercising, etc.Please see your doctor, talk to your therapist...It might be time for a re-evaluation.
Anything is possible. Having one problem doesn't mean you can't have another. Why not talk with your doctor about that possibility?
Concentration. Lack of concentration or inattention is only one symptom of adhd. Others can be disorganization, not finishing tasks, forgetfulness, misplacing things, overwhelmed with tasks, etc. Your doctor can reassess & give advice.
YES IT COULD BE. Plenty of good treatments, but you need seen.
Could be processing . Lack of focus can be from learning or processing problems. Medications will not fix those.