Moved Hyper-V 2012 VM to Hyper-V 2012 R2 long ping respond, slow connection

Hi,

I have two Hyper-V server, one in 2012 and the other in 2012 R2. I use SCVMM 2012 R2 and begins to move all my VM(host 2012) to my new host 2012 R2. Some of the VM begins to have slow network connection. I try to upgrade the integrated Windows services but not luck, some vm are really slow with the ping respond (over 75ms). Other VM who's been migrated and works well responds in 1ms like the host itself. a new vm directly created on the 2012 R2 host is fine and respond in 1ms. So what can be the problem?

Thanks