Experiencing Lack of Therapeutic Effect from ADHD Medication

Hi all, I was diagnosed with ADHD in approximately 2011. shortly after my diagnosis, I was started on Vyvanse. For a long time this medication has had therapeutic effects for me, and it is the medication I still take to this day. The problem is that I no longer feel a therapeutic effect from the med, and my concentration is all over the place. Despite this I continue to take the medication on a daily basis because if I do not I am extremely fatigued and can barely get anything done because I am sleeping all day. I do experience excessive drowsiness during the day even when taking Vyvanse, but not to the same degree as when I do not take it. I would prefer not to increase my dose (currently 50 mg). I guess my question is have any of you guys experienced similar symptoms? Does anyone have any suggestions? Any advice for meds to try? The obvious answer is switch medications, but because I have taken the med for so long I am apprehensive to try something new. I'd love to hear your thoughts