Question about payments for data sourcing and analytics payments.

[cnsrtmtrds]

Hello,

I started a new gig a while ago which is to give services for market analysis and sourcing. Which I present to the customers data in report and excel sheets. My problem is that sometimes after giving the data to my customer, he might claim that my report is not complete. The issue is that my research requires me a lot of work but sometimes I wont have data for some of the required fields because of the unavailability of the required data. How can I solve this issue with the customer? Especially when they refuse to approve my work and keep asking for revisions?
I was thinking about making payments into steps which might work but it might make customers go away. 
I am happy to here some ideas and opinions from other people. Thank you.