Help needed to understand Monitor.TryEnter (Object, TimeSpan)

I believe I have fundamental misunderstanding of how Monitor.TryEnter (Object, TimeSpan) works.  This is best explained by the following code:

private async Task MethodAAsync()
{
  if (Monitor.TryEnter(oLock, TimeSpan.FromMilliseconds(10000)))
  {
    Debug.WriteLine("Start A");
    ...
    Debug.WriteLine("End A");
  }
}


private async Task MethodBAsync()
{
  if (Monitor.TryEnter(oLock, TimeSpan.FromMilliseconds(10000)))
  {
    Debug.WriteLine("Start B");
    ...
    Debug.WriteLine("End B");
  }
}

I though the following output should never occur, but it is actually occurring:

Start A

Start B

...

End A

I am omitting try-catch blocks in the above code.

I am trying to prevent MethodBAsync() from running any code during the execution of  MethodAAsync().   Could anyone kindly point out my misunderstanding?

Hong