Co-registing multiple slc images Nan values problem

Hello snap’s world,
I am a beginner user of snap tbx1 and I’m trying to co-register 5 S1 SLC images together using the one master image of them, so I proceed like that:

  1. using a batch process I execute this graph on all the 5 images :

the results images are quite perfect : img1, img2, img3, img4, img5

  1. to complete the co-registration I use the graph belew with the last results for the entree:

But the problem is that the stack resulting from this process, contain just two normal images: the master and the slave1, but the other slaves(slave2, slave3, slave4) contain Nan values data.

notice that the images are perfectly superposed and chosen with this configuration:

But when I try to co-register them pair by pair that works perfectly

How can I solve this problem without processing each pair?