TUN interface problems

After creating a TUN device using C++ (used Kernel Documentation for
TUN/TAP driver) the device is set up correctly, with a correct IP
address, subnet mask etc.

When pinging from Machine A to the TUN Device, a reply is generated
and sent.
When pining from the TUN Device to Machine A, the request does not go
through. It is captured on the interface but the target machine never
receives the echo request. A correct route is set up for the target
machine.

When looking at the captured packet from wireshark, the following is
shown:
Linux cooked capture
Packet type: Sent by us (4) Link-layer address type: 65534 Link-
layer address length: 0
Source: <MISSING>
Protocol: IP (0x0800)

but when responding to an incoming ping, the "Linux cooked capture"
header part is correctly set up. Why is this?