Is there a 'dead zone' in meditation effort?

If you do something steadily for a long period of time you will personally see the "gains" in a rather slow manner. If, however, you stop doing something for quite some time and practise that thing again, the skill comea back quickly again. Once learned (in the brain), never able to unlearn. Unlearn is the key here. Neurons that aren't used become dormant. If a smoker hasn't smoked for a couple of years and starts again, the habit will be back quite quickly because the pathways were only dormant. If you want to become better at something you have to increase both quality and quantity

The stream flowing to Nibbana is entered by letting go (vossagga), as written at the end of MN 118 and in SN 48.9 and SN 48.10.

Most meditation taught (Visuddhimagga, Mahasi, Goenka, etc) are not based in letting go. Instead, they are "yogic techniques" based in mental manipulation.

Because these yogic methods use thought, the mind remains coarse or gross. Bodily formations only calm to a certain degree without the mind calming enough to bring more refined progress. Thus meditation "plateaus" or progress becomes "dead".

Yes, it has been pointed out in texts:

During the K'ai Yuan era (713-741) an ascetic Mazu was dwelling in the Ch'uan Fa Temple. All day he sat meditating. Zen Master Huairang knew that he was a fitting vessel of teaching, and went to question him:

“Great Worthy, what are you aiming at by sitting in meditation?”

Mazu replied, "to become a Buddha."

So Huairang leaned over and picked up two tiles off the floor and started rubbing them against each other.

Mazu asked, "What are you doing?"

Huairang replied, "Trying to make a mirror."

Mazu asked, "How can you make a mirror by polishing a tile?"

Huairang said, "How can you become a Buddha by sitting in meditation?"

Mazu asked, "Then what shall I do?"

Huairang asked, "When an ox-carriage stops moving, do you hit the carriage or the ox?" Mazu had no reply. Huairang continued, "Are you practicing to sit in meditation, or practicing to sit like a Buddha? As to sitting in meditation, meditation is neither sitting nor lying. As to sitting like a Buddha, the Buddha has no fixed form. In the non-abiding Dharma, one should neither grasp nor reject. If you try to sit like a Buddha, you are just killing the Buddha. If you attach to the form of sitting, you will never realize the principle."

Upon hearing this Mazu felt as if he had tasted ghee. He bowed and asked, "How should one's mind be so that it will accord with the formless samādhi?"

Huairang said, "Your study of the teaching of the mind-ground is like planting a seed. My teaching of the essentials of the Dharma is like heaven bestowing rain. Because you have natural affinity, you will perceive the Way."

Mazu also asked, "The Way is without form, how can it be perceived?"

Huairang said, "The Dharma-eye of the mind-ground can perceive the Way. It is the same with the formless samādhi."

Mazu asked, "Is that still subject to becoming and decay?"

Huairang said, "If you see the Way through such concepts as becoming and decay, meeting and parting, then you do not truly see the Way. Listen to my verse:

The mind ground contains various seeds,

Which with rain will come to sprout.

The flower of samādhi is formless,

How can it decay or become?"

Mazu was awakened and his mind became detached. He stayed to serve Huairang for ten years, gradually deepening his understanding of the profound mystery.

Buddhist Enlightenment is not something that can be achieved through mechanical repetition or by waiting, like one would wait for a kettle to boil or for a train to arrive. Buddhist Enlightenment involves shedding layers of bullshit and meeting reality. This is a process that requires courage to go forward into the unknown without a map. Only then one's practice can become alive and one can meet the real dragon face to face.

This includes meditation, that has to get less and less scripted and more and more an authentic presence and discovery with no preset agenda.