The vacuum worked at first, but after six months started to malfunction. This is what happens with every use which prevents us from using the machine. On low-nap carpet with height set on 6 (one below highest height), after moving forward and backward one time rolling freely, the vacuum's suction causes the vacuum to stick to the carpet. The vacuum is much too hard to move. It does not roll freely. After this happens, turning the motor off and back on does not affect this problem. The height adjustment knob does raise and lower the body, but the body seems to fall lower on height setting 6 after starting to vacuum. On the same carpet, the brush does not touch the carpet when the height is set to 7, the setting for highest carpet. When the height is set to 5, the machine can be rolled with difficulty, but sucks the carpet off the floor. What has happened to cause the vacuum to malfunction like this? What can I do to fix it?