how to debug losing nfsroot connection on Centos 7 ? (observing "task blocked for more than 120 seconds")

I am experiencing diskless clients losing connection to their nfsroot server within 24 hours of booting. Initially I thought it was hardware related as i simultaneously upgraded 16 blades from Centos6 to Centos7 (diskless/pxe boot with nfsroot) and they all lose connection at the same time after booting ok and running 12 hours+. When they do they all print to the console "task blocked for more than 120 seconds". I setup one of the blades to boot from local disk and when reproducing the problem the 15 diskless blades fail as described and the blade with boot disk continues as before. The nfs server continues serving other clients fine. I've concluded that my nfsroot connection is getting lost on these diskless blades (Dell M620s in M1000e chassis). Nothing interesting is getting logged in messages file either end. I do not think it is hardware because the all that's changed is upgrade from Centos6 to 7, although there could be compatibilty issue i suppose. The hardware does claim to support Centos7. Can anyone advise good way to debug why the nfsroot conenction is getting lost ? kernel = 3.10.0-1160.59.1.el7.x86_64