Why are my onion bulbs not forming?

Onion bulb formation is triggered by day length. Long-day varieties (common in temperate countries) will never bulb in India's shorter days. Always use short-day or day-neutral Indian varieties: Pusa Red, Agrifound Dark Red, NHRDF Red. Other causes: excess nitrogen keeps plants vegetative — reduce nitrogen feeding from January onwards; pot too shallow (bulbs need 8+ inches); or harvesting too early. Bulbs form only from March onwards in North India.