You may have the best voice in the world but what if you have ....some difficulty that is not allowing you perform your best, fortunately most of them you can control.
Pertaining to any structural deviation or damage to Vocal cords and larynx as a whole , local pressure effects due to thyroid or parathyroid compression on the Larynx etc.,
Dehydration, excessive usage of vocal cords, poor physical conditioning.
Anxiety, Stress, Nervousness(Neurosis),depression, psychosis, insomnia
Various muscular dystrophies
Neurological damage or impairment:
Motor Neuron Disease, Parkinson's disease, stroke etc.,
Medications and other drugs:
A lot of medications that we use cause dryness of mouth due to their parasympathetic affects.
Excessive Caffeinated beverages can induce, provoke or exaggerate anxiety, Alcohol can cause dehydration, Smoking is the other dangerous habit that can ruin the voice.
So if you are not able to sing to your potential you might be dealing with one or more of those problems. So get your self evaluated if you think you might dealing with any of those and try to avoid those factors that can impair voice and performance.