How can WriteFile succeed without writing any data?

I'm calling WriteFile to send data to a modem:

BOOL writeResult = WriteFile(m_hPort, p_message, length, &numOut, NULL);

where:

m_hPort is a valid HANDLE
p_message is an unsigned char* containing ate0\r
length is an int with a value of 5
numOut is an unsigned long initialised to 0

Occasionally I'm seeing this method succeed but numOut != length

How is it possible for WriteFile to return success without sending any data?

Edit This is how I'm creating the handle:

HANDLE hPort = CreateFileA("\\\\.\\COM5", GENERIC_READ | GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, NULL);

I've checked the return and it's not INVALID_HANDLE_VALUE which suggests it's valid.

Solution

Turned out to be a hardware issue after all.

Power cycling the converter would clear the hardware buffer leading us to suspect that it was a problem with the drivers for that device. From Hans' suggestion we dropped the timeouts and then started probing the error reports in more detail.

Reducing the timeout to something sensible meant we were able to identify that the buffers were filling up until they cannae' take nae' more! Which is why power cycling the converter fixed it (we tried re-routing power through engineering and reversing the polarity, but unusually, neither resolved the issue).

The root cause was some flaky hardware flow control causing the software to block indefinitely. Disabling the flow control solved the issue.