Forms Responses not including images

I have created a simple form of four questions to capture delivery ticket information. 

The last question asks the user to upload an image of the delivery ticket.  This prompts the user to open the camera and take a picture. This form is being completed on a Samsung Galaxy tablet.   

For some reason (even though the question is marked as mandatory) when I open the excel sheet which summarises the data, the link to some of the images is missing.  This is very random, as some image links are present.  What could be preventing some of them uploading?