I have a Dyson Ball Complete (purple) upright and the thing is impossible to push on a carpet. How do you adjust the height of the vacuum for various surfaces?

SOURCE: Hoover Windtunnel carpet level adjustment

The suction is so great that it sucks up your carpet, then nothing works. I get my children to stand on the corners/edges of the rug so that it stays perfectly flat on the floor while I vacuum. Or, you can turn off the brush roller (set it to "hard floor" setting). Or, tip the vacuum cleaner's front up slightly while vacuuming, to reduce suction.

SOURCE: dyson - can't push the vacuum

try to see if the carpet setting is to carpet. i know this machine says that its a life time belt,but i would check this just in case. best way to tell if it's working is to lay the machine on its belly with the brushes pointing up and turn on the machine to see if the brushes are turning. (be careful not to put your face directly in front, just in case something is lodged in the brush and flies out )

SOURCE: motorized brush bar not turning on Dyson Vacuum

Apparent common failure is that either the thin yellow and or blue motor drive wires that run from the LH side of the vacuum where the "control board" is located down into the hinge / ball area become stretched and then fray and eventually break. Happened to my Dyson twice. First repaired under warranty, second took about 20 minutes. You need a Torx screwdriver to remove the LH panel ( 4 screws) and also an access panel (2 screws) at the side of the ball which permits access to the blue / yellow wire connectors. I replaced with a new wire and connectors having fed the wires through the access hole making sure sure enough wire was available for the movement of the handle. Also ensure that when the covers are replaced they do not trap any wires underneath.

SOURCE: Bissell Pet Hair Erase does not vacuum.

It is because the belt is worn or stretched and needs to be replaced. Here are the belts you need and the manual which has instructions:

Bissell Pet Hair Eraser Belts and Manual

SOURCE: Smartstrand carpet makes vacuums difficult to push

The seam problem could be an installation problem. It is important to pattern match any carpet with a pattern. If the carpet is a plush carpet, the seams should not be invisible, but not unsightly. If he carpet is a frieze, the seams should be hard to find. Any variance from these norms could mean a redo of the seams by a talented carpet installer. In a large room such as yours seams will never be invisible.
Good Luck, carpetguru