How many NICs per server do you have? and
What kind of NLB is it (multicast / unicast) ?
It can happen if you use the default NLB type "unicast" while having just one network adapter. It is because the heartbeat between the servers won't work because they understand that it is a loopback sending and receiving from the same mac address and each server won't know the status of the partner
If it is your case, that you have just one connected NIC per server. You shall consider using "multicast" NLB "if possible" or even better... add a new network card for communications between NLB member servers and their administration
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Another reason that could generate the problem is the switch. If you have an advanced switch with built in security configured and active, it won't allow that you use the same mac address from two different ports (like in Unicast array), and it will drop one of the connections or it can block multicast traffic for avoid sending communication to multiple ports at the same time (like in Multicast array)
If it is the case you can communicate with the person that configure your routers and they will asist you and inform you what kind of NLB shall you use
