Alice Bailey wrote a book A Treatise on White Magic which is all about the process of manifestation of thought-forms on the physical plane.
It begins with a seed thought on the mental plane which attracts mental elementals to create a thought-form. This requires clarity and concentration to create a potent thought-form. If the thought-form lacks potency then it will fail to develop.
The thought-form then descends to the astral plane where it attracts astral elementals of resonant vibration, which gives the thought-form vitality and magnetism. This requires a calm astral (emotional) body - too little astral energy and the thought-form will wither away. Too much emotional energy and the magician loses control of the thought-form.
This thought-form then descends to the etheric levels prior to physical manifestation. Here the magician needs to be sensitive to the conditions of the etheric field and the sense of timing. Are the etheric conditions right for the thought-form to manifest?
If all is done correctly then the thought-form manifests on the physical plane. But at each stage of the process (mental, emotional, etheric) the magician has to control the process and provide the right conditions. Otherwise the thought-form gets so far and no further.
Alice Bailey also mentions the devas of circumstance. These are devic beings who respond to the intent of the magician. The higher the intent, the greater the response from the devas of circumstance. So things happen to ease the process of manifestation, such as apparent coincidences or things happening with perfect timing.
In short, if people fail to manifest something then it is usually because they lack the required ability to function effectively mentally, emotionally and etherically. Some people's attempts at manifestation are little more than wishful thinking with no real force or focus, and then they wonder why they fail.
Just one perspective.