Firmware updates fail on certain subnets (sites)

Newbie question, sorry.

I have had to change my Wyse FW deployment method from FTP to Http recently.

FW updates work fine to only a limited set of subnet (or sites, as subnets are different per physical location). DHCP options 161,162 are exactly the same on both sites (subnets) and all point to the same IS Server in a central location. Info in the IIS Server logs is limited, but I can see both the working and failing WYSE devices connecting, but no FW file download on the failing subnet devices.

WNOS.INI file is the same, and set to enable either upgrade/downgrade. Devices are R10 and C10’s. Can anyone offer any advice ? . I have spoke with perimeter security guy, and he assures me there is not firewall restriction difference across sites.