sounds like the top burr carrier isnt being seated properly.
Twist the collar around (coarser) until it wont go any further, then press the button on the back and keep twisting until the arrow on the burr carrier lines up with the arrow at the back of the grinder. This should enable you to take the collar assembly off altogether.
Now take out the top burrs (there should be a small metal handle on the burr carrier that you can use to twist the burrs) by turning them and pulling at the same time.
Now, place the collar back on the machine so that the arrows line up again. Next place the burrs in the collar and then rotate with gentle downward pressure, they should then fall into place. Do not continue to try and turn the burrs. On a new machine, there should be arrows / marks on both the burr carrier and the collar that line up when the burrs have slotted in correctly. Now turn the entire collar back round. This should have the burrs seated correctly into position. You should be able to look into the top of the burr assembly and see the gap between the top and bottom burrs widen / shrink as you make the grind coarser / finer. The movement wont be much each click, but a few clicks should make a visible difference.
Some other possibilities / thoughts -
Are they fresh beans (from a local roaster, not the supermarket)? If from a supermarket, they will not be fresh and you will struggle to get any sort of decent espresso shot and / or consistency of grind.
How much coffee do you have in the hopper. The 480 likes to be roughtly 1/2 full of beans to provide adequate downward pressure on the beans as they pass through the burrs.
Is your dosing / tamping routine the same for each grind? When trying to get a decent shot, and something goes wrong, only change 1 thing (usually the grind), keep the dose (amount of coffee) and the tamp (pressure) as close to the same each time as you can.
Are you getting the same level on the 6910 tamper each time (1/2 of the steel above the rim of the basket).
Have you taken out the pressurised filters from the PF and using the standard filters?
The pair of the 6910 / 480 is capable of making wonderful coffee when paired with a little knowldge, a lot of passion and (some say) most importantly FRESH BEANS