I ran into the same problem the first few times I tried it. It is better to have someone to help you line up the centre of the car to the centre of the dolly. I put pieces of tape on the car and dolly to aid the spotter. It is very difficult to do this yourself as you are not sitting in the centre of the car and can't judge it very well.
You should make sure that both tires are as close as possible (within 1/2 inch) to the vertical bar with at least one touching. You will find that when you check the car after driving it for a few miles that at least one of the tires will be a few inches back from the bar and no amount of tightening it will close it up. This is normal but since the straps will stretch when new they should be checked and tightened after a few miles.
When you get to your destination unless you are able to come to a stop after driving in a straight line for several feet then one tire will be up against the bar and the other one will be several inches back. This is no problem if you have left enough slack in the safety chain going over the suspension. If the safety chain is tight on one side then you can't fix it by driving the car forward. You must swing the dolly in the direction of the gap.
If the right tire is moved back from the front lip, you can't fix that by trying the drive the car forward. Instead you must swing the tongue of the dolly to the right. To do that, turn the wheels of the towing vehicle to the right, and back up a few inches. Reverse that for the left car tire.
Gary & Nancy
2001 Triple e Commander A3509FB - Ford F53
2010 Toyota Matrix