Re:
There are more threads in there! Try a slightly longer bolt with a few more threads. You should be able to catch more threads inside. Don't go too much longer or you risk bottoming out and pushing out the top, which will show on the top of your mount (test carefully). When you tighten, you don't want to go too tight when only catching the few remaining threads. Having said that, those clamps don't need much to hold the bar adequately.
Good luck!