1. Full Spectrum Analysis
2. Packet Capture
3. Someone who knows how to interpret Spectrum Analysis and Packet Captures
Your issue could be a large number and combination of things. If you have the same settings on the controller and access points at other locations, it is most probably environmental. Warehouses are the most troublesome environments to deploy and then maintain.
My advice is to hire a professional who is used to dealing with these issues specifically, and that will allow you to get to the heart of the problem in the least amount of time. It ls literally impossible to troubleshoot warehouse problems conclusively from a forum.