Nothing is wrong. The cap is small by design. As you can see from the picture below the cap sits very low in the capsule without any coffee.
The cap is not meant to be a tight fit or close to a tight fit. It is designed to be used with a filter which comes in each package as a sample and we also sell the filters by themselves.
We originally did make the cap a tight fit, but what we found was that it put too much pressure on the outside walls of the vertuoline capsules causing them to get stuck in the machine. We got a lot of complaints about that. We found that by making them loose it did not push out on the capsule.
The cap is meant to be used right away. So it being a little loose does not matter. If you want the capsule to be completely sealed, then we suggest that you use our foils. The are a great solution if you plan to make a bunch of them in advance and want them all to be sealed.