I'm not sure, as from my understanding, this treatment is usually effective at lessening fatigue. Have you had your TSH levels checked recently? Treatment with levothyroxine may be needed on a long-term (lifelong) basis, but your dosage requirements may change. It may be as simple as needing an adjustment in dosage.
Have your symptoms recently returned, or have you found the medication to be ineffective throughout the year? Do you have other symptoms that have/have not improved since being diagnosed and beginning treatment?