Hyper V Virtual Ethernet Adaptor loses default gateway server 2012

When I create a virtual switch on hyper v, i lose network connectivity and the default gateway is missing
I have to manually put the default gateway in and disable and re enable the network adaptor each time the server restarts in order to get network connectivity. I am running windows server 2012

I have tried deleting the virtual switch on hyper v, deleting the network adaptor and reinstalling it and creating the virtual switch again

Any help would be great

Thanks