GPIB - when I use wait with a timeout of 1sec, the wait function doesn't return after 1sec

I use the following code:

gpibDevice.Wait(GpibStatusFlags.DeviceServiceRequest | GpibStatusFlags.Timeout);

I want to wait for ether a DeviceServiceRequest or a Timeout. When I use a Timout of 1sec

the wait function doesn't return after 1sec. Can someone help me, why the timeout is not effective here????????