Weird Network Problem HP 5500 HI

[colly72]

Hi all,

We have 2x HP 5500 HI switches, in IRF mode, as our core switches.  They've been working away happily routing our VLANs for the last few weeks, since we installed them.  Half way through this morning, we suddenly were unable to connect to some devices on one VLAN.. Nothing has changed on the switches, in terms of config, or on the rest of the network.  As soon as I put the devices on another network, they can be seen.  It's not a routing issue, as some devices work OK, nor is it an ACL thats blocking it.

Can anyone help?