EMD function for IMFs of a signal

Hello there,

I'm using the EMD matlab function for my thesis. I need to decompose a certain signal into IMFs and I'm noting an issue regarding the EMD function output: the problem is that it seems to me that each time I run my script, containing the Matlab EMD function, that function gives me back different IMFs with respect to the previous time. Why does it happen? How can i fix it?

Thanks in advance.