Problem with VMs connecting via Cisco Nexus 5548UP.

I am having an odd issue with some ESX VMs connected to a Nexus switch. Here's a simple breakdown of what's happening:

Host A - has a virtual switch with it's physical NIC connected to Switch A. Switch A is connected to the rest of the network.

Host B - has a identically named virtual switch, with it's physical NIC connected to Switch B (the nexus 5548up). Switch B is connected to Switch A for it's connection to the rest of the network.

While the VM is on Host A, all is fine, everything works as expected. If I move the VM to Host B, I can still ping everything I can think of (internal and internet address alike), I can access most things, but some things will not communicate. For example I can connect to some servers via port 443, but others I can't. With the VM on HostB (and on the Nexus switch) I can access our internal sharepoint (which uses https), but I can't access the vSphere web client on any of the ESX hosts, yet I can access it on vcenter. Once signed into vCenter through the VM on HostB, I can for example browse the datastores, but I can't download any files from the datastores. Yet if I migrate the VM back to HostA (using Switch A) everything works perfectly fine.

I can't find anything wrong with the config, there is no internal firewall at play. The only difference between the 2 hosts is HostB has it's virtual switch connected to the Nexus, which connects to switch A. Where as Host A connects directly to switch A.

Has anyone had this (or similar) issue before? Any suggestions?