Hi there,
Not sure what the OS is for your vMA but a couple of things you could try:
- shutdown the vMA and if the option is available, right-click and select Upgrade Hardware
- shutdown the VM and then remove the current adapter and re-adding another.
It may be that the vMA was created with an older version of ESX and specified the VMXNET adapter whereas the new version of ESX has branded this as Flexible.
Make a clone/backup of the vMA before doing this if it has important stuff on it.
Kind regards.