Why are my marigold plants flowering very little and growing only leaves?

Lots of leaf and few flowers in marigold usually means too much nitrogen, too much shade, or wrong season and daylength for the variety. Heavy urea or rich manure pushes leafy vegetative growth at the cost of flowers, so balance the nutrition with phosphorus and potash and ease off nitrogen. Make sure the plants get full sun, since shade delays and reduces flowering. Some varieties flower poorly when the daylength or temperature is not right, so use a variety suited to your planting season. Pinching early helps branching and later flushes. If pests like the larvae that bore buds are eating the flowers, that also looks like poor flowering. Check your fertiliser plan and variety choice with your KVK.