Hi
I did this randomly with 1 person whom I had nothing bad.
I called but it was not answered. I just had a twitch to check if it works..
I visualized a call of that person, name flashing on mobile phone
only once. 

I got her phone call within 5 hrs asking if all is good with me?
She is having some stomach pain and bla bla....
With specific person, we have resistance, we want it = living in lack 
and lots of negative energy in it, needs to be cleared for this SP phone call

But randomly keep trying, you get to know where you stand in manifestations!
few cases are tough, few are simple. I have seen at my Guru's place since childhood
like spiritually treating for a simple headache, person gets some blessed thing by Guru
and it's gone! But to survive a person in ICU is tough, lots of negatve energy. 
So, don't worry if it takes time to get your ex back. It is also negatiivity in this already 
you have created. 
So, key thing is ...
keep living life like this, practice things on every aspects. You get those manifestations and
it raisies your confidence. you understand more energetically
Master your thoughts, it is always working!