Several things can go wrong and cause neurotransmitters to not work as they should. Some of the prominent causes include: 

• Over or under production of one or more neurotransmitters
• Too much or too little of one or more neurotransmitters are released
• Faulty functioning of the receptor on the receiver cell, which hampers the ability of the normal-functioning neurotransmitter to effectively transmit the signal 
• The synaptic cleft may be inflamed or damage, hampering the cell receptor’s ability to take up enough neurotransmitter
• Enzymes obstruct the neurotransmitters’ path, allowing only a few to reach their target cell
• Neurotransmitters are reabsorbed too rapidly

Some of the underlying factors that cause neurotransmitters to malfunction may include: 

• Prolonged periods of stress
• Genetic factors
• Consistently poor diet
• Hormonal changes
• Faulty metabolism
• Intake of toxic substances such drug use, heavy metals, and pesticides
• Certain prescription drugs
• Alcohol, nicotine, and caffeine