Tadalis SX 20 mg contains tadalafil, the same active ingredient as other medications for erectile dysfunction (ED), and is generally effective in treating ED. However, there are several reasons why Tadalis SX 20 mg might fail to work as expected:

1. Incorrect Dosage: The 20 mg dosage might not be suitable for everyone. If the dose is too low or too high for your specific condition, it may not be effective. A healthcare provider may need to adjust the dosage based on your individual response.
   
2. Lack of Sexual Stimulation: Tadalafil (the active ingredient) requires sexual arousal to be effective. It does not work automatically, so if there is no sexual stimulation, the medication may not produce the desired effects.
   
3. Alcohol Consumption: Excessive alcohol can impair erectile function and interfere with the effectiveness of Tadalis SX 20mg. Alcohol relaxes blood vessels and may counteract the benefits of tadalafil, making it harder to achieve an erection.
   
4. Underlying Medical Conditions: Chronic conditions such as diabetes, high blood pressure, heart disease, or neurological disorders can interfere with the drug’s effectiveness. Poorly controlled blood sugar, for example, can damage nerves and blood vessels involved in the erectile response.
   
5. Medications and Drug Interactions: Certain medications, including nitrates (often used for chest pain), alpha-blockers (used for high blood pressure or prostate issues), and some antifungal or antibiotic drugs, can interact with tadalafil and reduce its effectiveness. Always inform your healthcare provider of all the medications you're taking.
   
6. Psychological Factors: Anxiety, depression, stress, and relationship problems can all negatively affect sexual performance. If psychological factors are influencing your ability to get or maintain an erection, Tadalis SX might not work effectively.
   
7. Poor Blood Flow: If you have significant vascular issues or atherosclerosis (narrowing of the blood vessels), blood flow to the penis may be inadequate even with the use of tadalafil. This can reduce the effectiveness of the medication.
   
8. Kidney or Liver Issues: If you have liver or kidney disease, the body may not metabolize Tadalis SX properly, reducing its effectiveness. Tadalafil may accumulate in the body, leading to increased side effects or insufficient therapeutic effect.
   
9. Counterfeit or Expired Medication: If the medication is not from a reputable source, or if it has passed its expiration date, it may not work properly.
   

If Tadalis SX 20 mg is not working for you, it's crucial to talk with your healthcare provider. They can help identify potential causes, whether it be dosage, underlying health issues, or other factors, and make adjustments as needed.