Error in stack process while doing subsidence

I am Doing displacement mapping by using sentinel- 1 data of the years 2017 and 2018( SLC - VV Polarisation). I split the image to my extent and I got Orbit files correctly for both images. But, While I am doing stack process (S1-TOPS Co-registration–> co-registration ) the output is not coming properly. I have tried S1-TOPS Co-registration–> co-registration–>Back geocoding and DEM assigned co-registration also. Stack for 2017 image(Master image) is coming properly but, 2018(Slave image) is not coming. A blank gray coloured image is opening for all i , q and intensity bands for 2018 imagery.
what is the problem in this?