Decreased focus. Metadate or Ritalin (methylphenidate) enhance focus in people with add. The effects last for a few hours and then fade out. If you miss a dose you won't have the benefit of the drug that day.
Two possibilities. The first is that you may feel restless, have difficulty focusing and distractibility. Second, if you have been on Ritalin (methylphenidate) for fatigue, you will likely become tired when you miss doses. If you have taken medication for a long time, missing an occassional dose may leave you tired until you restart the medication.