This is not something that can be answered without more info. What exactly means “uploaded by FTP”? Do you upload the files and then manually insert the address into the Source Code? What happens if you / the client directly tries to access the files by typing in the URL into the Browser Address Bar?
If you can remote into a computer at your clients office, take a look at the developer console. Are there loading or Server Errors in the Network Tab?
The possibilities are endless…