How are you trying to access the EWS for these printers? IP Address or Host Name? For example, I can access the same printer by going to https://192.188.1.5 or https://HP-Network-Printer. One relies on IP Address alone while the other relies on DNS to do the IP Address lookup behind the scenes. If you are unsure what your Host Name is then print off a Configuration Page from the control panel of the printer and review.
Try both techniques and see if you get any different results.
I have seen instances where the DNS values are out of synch which makes the EWS unavailable from the network. The best thing to do in those cases is reboot the printer and allow it to re-register with the DNS automatically. You can also perform Cold Resets on the printers when the networking information is not completly wiped out. Cold Resets are often the only way to remove a cached IP Address on a machine and allow it to auto consume a new DHCP IP Address.
The IP Address should be unique and work every time. If two pritners had the same IP Address neither one would work. The only way an IP Address could not access the EWS is if the printer actually had the EWS disabled. If that is the case then perform a Cold Reset to restore the functionality. EWS should always be enabled by default.