The cause
Lead generation has many natural and artificial limitations. You can reach a demand ceiling in a marketing channel, choose a poor marketing mix that won't get your target audience, or send a message that doesn't make sense to them.
The most common reason is a lack of understanding of the customer journey and the associated offering of the wrong content with the lousy call action at the wrong time. In turn, this lack of knowledge results from insufficient analytics of the digital footprint, human behavior, and motivation.