Please take a look at the ProcessingSteps To create SingleInterferogram in the following post,
Be advised to select SRTM 1sec DEM, in the step of Backgeocoding
In case still gap case, alret the ESD parameters:
Search Windows Accuracy in Azimuth direction to 32
Search Windows Accuracy in range direction to 32
Windows overlapping factor to 256,
But first apply the default with the aforementioned steps.