Interesting that you have the choice there about not realizing the problem(s) that they have. This is very real and can be one of the most difficult objections to overcome.
There are the obvious answers to this like competition, price, product offerings, delivery and other components but the reality could be that the prospect is not grasping the necessity for the product. My recommendation for this is to let the prospect do the talking on what they feel are their problem and see if your product or service fills a need there. I would love to hear from others on this.

By the time Prospects send up their recommendations for approval, they have moved on to other things. Prospects forget if the in the internal buying process takes too long.

In my opinion both the reason support why prospect go dark . For example, if they don't think they have a problem, so they do not followup and look for potential complications. Thus, with time pass by motivation died downs. Besides, when one get busy, sooner or later one will forget about the prospect as our life is occupied with so many stuff. What do you think?

Paolo, i am assuming your prospect means client project over here. (if it is not, please let me know what is your definition of the prospect you mean). Just my opinion: besides busy and problem, some time prospect goes dark because there is a lack of clarity. Clarity can comes in different aspect. For example, the clarity of setting the boundaries of the project dates and time for kick start, follow up and termination date, clarity of project investment capital and funding source, clarity of task responsibility for different member, clarity of the objectives, the vision and goal of the project. All these could be summarized under clarity.

" They don't think they have a problem". People perspective are different. One could see it as a problem while others see it perfectly correct! Take a look at this article (https://blog.hubspot.com/sales/qualified-prospects-go-dark) that could provide some insight why prospect go dark and how to solve it! I think the next step would be how do you solve it after you know that your prospect go dark!

Various reasons -

Priority
Priority is a major issue. Often the priority gets changed by the business ( The team which is paying for the prospect to be an account).

Lack of clarity or no visibility
They don't have clarity or visibility of the problem.

Cost and Vendor
Cost is a major factor too Or you may be not the right vendor. Client might be shopping around :)