Jesus spoke of prayer having the power to move mountains. Speaking to the individual he said that a person with faith as small as a mustard seed can do this.  Read Matthew 17:20.  A cynic would say that in those times where everybody died young and people saw death as a welcome escape to a paradise it was not going to be very testable.

He did not qualify this by saying it moves the mountains in the heart that stop you being a better person.  If he did it would be the difference between saying you really can move mountains in the world where you can take credit for the COVID vaccine simply because you prayed and blaming yourself for not being perfect inside.  Experience shows that prayer does not remove spiritual and psychological mountains.  It cannot do much if anything for what you are like inside.  A person trying to tell you that prayer will change you is to blame if you depend on that help and nothing happens.  The serial killer who wants to stop and depends on prayer will go among people and to his horror end up having committed another murder.  The quacks led the way.

If several people pray for something and it happens that does not mean that they all got a result.  Maybe God was only interested in your prayer.  Notice how Jesus keeps it about you not people in general.  This leads to extreme pride and arrogance.

The danger with his teaching is that he is telling you that even if you cannot see it your prayer has done immense things. This makes the person of prayer an individual who uses the misery of others as an excuse for praying and thinking that they have done something when they have done nothing. And not only that. That they have done amazing unseen things. Action if any is only a small part in this amazing framework. In reality it is nothing and it is only prayer that makes work mean anything.

As we say, there is no proof that you have shifted a mountain.  God must want you to have faith in yourself when he hides this marvellous transformation in the situation brought about by prayer. Jesus knew a thing or two about DARVO.  DENY.  ATTACK.  REVERSE VICTIM AND OFFENDER.  He was blaming his victim for his promises not coming true.  Either the victim was not really praying sincerely or the victim is evilly denying the good work the prayer has done.