DNS Zone transfer fails to windows NS

[mobcdi]

My master ns is running cPanel but it is refusing zone request transfers from the secondary ns which is running windows so my serial numbers are out of sync.

Since clustering isn't supported on non cPanel servers, how do I go about allowing the secondary ns to be updated?

I read that all you need to do is include the 2nd ns in the zone file but I have done that

cdi.ul.ie. 86400 IN NS source.cdi.ul.ie.
cdi.ul.ie. 86400 IN NS mercury.ul.ie.

and the secondary NS (mercury.ul.ie) is still saying that transfers from source are not allowed
"A zone transfer request for the secondary zone cdi.ul.ie was refused by the master DNS server"