Yoga Sutras 1.30
Patanjali, explained by Swami Vivekananda
व्याधि-स्त्यान-संशय-प्रमादालस्याविरति-भ्रान्तिदर्शनालब्धभूमिकत्वानवस्थितत्वानि चित्तविक्षेपास्तेऽन्तरायाः ॥
vyādhi-styāna-saṃśaya-pramādālasyāvirati-bhrāntidarśanālabdhabhūmikatvānavasthitatvāni cittavikṣepāste'ntarāyāḥ
Disease, mental laziness, doubt, lack of enthusiasm, lethargy, clinging to sense-enjoyments, false perception, non-attaining concentration, and falling away from the state when obtained, are the obstructing distractions.
In plain words: The things that block a yogi are: illness, a lazy mind, doubt, lack of enthusiasm, heaviness, craving pleasures, seeing things wrongly, not being able to focus, and losing focus after finding it. The teacher says to care for the body and keep practicing when doubts come.
Swami Vivekananda explains
Disease. This body is the boat which will carry us to the other shore of the ocean of life. It must be taken care of. Unhealthy persons cannot be Yogis. Mental laziness makes us lose all lively interest in the subject, without which there will neither be the will nor the energy to practise. Doubts will arise in the mind about the truth of the science, however strong one's intellectual conviction may be, until certain peculiar psychic experiences come, as hearing or seeing at a distance, etc. These glimpses strengthen the mind and make the student persevere. Falling away … when obtained. Some days or weeks when you are practising, the mind will be calm and easily concentrated, and you will find yourself progressing fast. All of a sudden the progress will stop one day, and you will find yourself, as it were, stranded. Persevere. All progress proceeds by such rise and fall.