Could also be bad propagation. There are good days for SW, and bad days....
Over the past few weeks there have been nights I haven't even heard WWV. Not on 10 MHz, 5 MHz, 15 MHz, and sometimes even 2.5 mhz. And that's weird.
ONe thing you could do is check MW. If the radio isn't receiving MW using that loop, then there may be something wrong with the connections, which range you have it switched to, etc.
The other guys have good ideas... high up, and near a window, doublecheck the connections, make sure you're tuned to the correct ranges... and remember to tune above 9-10 MHz during the day and below 10 mhz during the night, you'll probably get better results.