Whenever I am just standing with my bike with the front brake only locked, the bike is able to move forward and backwards slightly if I push it. The tire is not moving on the ground and the brakes arent slipping so I am pretty sure it is the headset. When I put my finger on the head tube such that it is touching both the frame and the headset I can feel it move a little bit when I rock it. How do I tighten my headset??? I have tried loosening the stem from the steerer tube and then tightening the adjuster bolt on the top of the headset but it never seems to get any tighter once I retighten the stem. I know this is probably a pretty basic problem but I dont know how to get this thing tight. Can anyone help me??? Thanks.