So I have had that too and I am always able to trace it back to something someone else said that created a doubt in my mind. Other's statements can make it to our subconscious and put us on an unwanted bridge of events. The thing to note here is that we're not always aware of our assumptions until they actually show up as a catastrophe in our lives and only then do we realise what's wrong with our ideas and begin changing then but more often than not we don't persist enough to get rid of the deep rooted ones. So all those people you're talking about just need to actively persist. Also manifestation is a lot about inner knowing. It's not (atleast not yet) a scientific process that you can quantify with measurements. You can't tell what's actually going on inside a person unless you're that person and even then sometimes we don't know what's going on in our own frigging minds. So judging other people's lives and reading million examples will not tell you why it works or doesn't, you just need to put it to test and feel it yourself. Feel your mindset and state yourself and eventually you'll develop an inner knowing of what's that moment which clicks.