Patanjali Yoga Sutra — vyādhi styāna saṃśaya pramādālasyāvirati bhrāntidarśanālabdhabhūmikatvānavasthitatvāni cittavikṣepāste’ntarāyāḥ||1.30||

व्याधि स्त्यान संशय प्रमादालस्याविरति भ्रान्तिदर्शनालब्धभूमिकत्वानवस्थितत्वानि चित्तविक्षेपास्तेऽन्तरायाः||

The impediments or hurdles to knowledge are: disease, intellectual sloth, suspicion, absence of zeal, physical lethargy, hankering for sensual enjoyment, false views, and distress due to unsuccessful concentration and shaky concentration.

Having said that repetition of AUM helps the Sādhaka to overcome the obstacles in Yogamārga, now Patañjali Maharṣi explains the nine obstacles (enemies of Yogasādhaka) in the path of Yoga sādhana.

1. Vyādhi: āyurveda says imbalance of tridoṣas viz., vāta, pitta, kapha is Vyādhi (disease) and thus, one is not able to do Yogasādhana. Vyādhi could be either ādhija vyādhi or anādhija vyādhi. According to the traditional Hindu view, to suffer from any disease is sin. One suffers diseases only because of one’s own idiocy. Disease, suffering, etc are not in nature. In the great epic Mahābhārata, vicitravīrya, son of śantanu and Satyavati, suffer from diseases purely because of his own polluted lifestyle and dies at a very young age!
2. Styāna: Incapacitation of mind to understand what is stated in yogagranthas, purely because of mental sloth.
3. Saṃśaya: Doubting, suspicion, skeptical attitude towards what is said in Yogic scriptures.
4. Pramāda: scanty attention, negligent attitude towards Yogasādhana.
5. ālasya: Laziness, indolence, lethargy (of the body) and not being able to do Yogasādhana.
6. Avirati: Restlessness due to inability to control sensual craving, thus not being able to do Yogasādhana..
7. Bhrāntidarśana: Illusion, delusion, fantasy, inability to gauge one’s own limitations or boundaries and thus undue overstretch in sādhana.
8. ālabdhabhūmikatva: Not being steadfast, inability to reap and realize the benefits of Yogasādhana. śiraḍi sai bābāji was asked a question. “Will everyone visiting śiraḍi for your blessings get liberated?” bābā answers — “Look at this huge tree. Millions and millions of flowers are there. How wonderful it would be if all were to become fruits!”
9. Anavasthitatva: Inability to retain or sustain what is achieved through one’s own Yogasādhana due to the troubled condition of the citta.

Enough stuff is available in Bhagavad Gītā in this aspect and Bhagavān touches upon some more impediments in the path of yoga as follows:

नात्यश्नतस्तु योगोऽस्ति न चैकान्तमनश्नतः।

न चाति स्वप्नशीलस्य जाग्रतो नैव चार्जुन॥६.१६॥

nātyaśnatastu yogo’sti na caikāntamanaśnataḥ.

na cāti svapnaśīlasya jāgrato naiva cārjuna ||Gītā 6.16||

Yoga is not for those who gorge too much, nor for those who eat very little and starve. Also, success in yoga is not possible for those who sleep too much, or for those who do not sleep enough. That means moderate, or being temperate in every action alone aids Yoga sādhana.

The basic nature of the body is always being healthy. As long as external or internal (mind) factors do not affect, the body ever remains healthy. The basic nature of mind is always being calm. When left on its own, undisturbed, mind ever remains tranquil. Thus, when there is a small imbalance in the health condition, the body tries to correct on its own to restore the original healthy condition. The basic nature of senses (pañcendriyas) is always to remain subdued. Unless external and internal factors cause to bother, senses ever remain composed. The basic nature of prakṛti is always being serene and balanced. Unless other factors affect (for example: interacting triguṇas), prakṛti ever remains unruffled.