Invisalign is a ground-breaking orthodontic appliance that shifts your teeth into the right position without the use of metal braces. The entire system is custom tailored for you and made up of clear (transparent) plastic ‘aligners’ that fit your teeth snugly and straighten them.
The Invisalign trays are created by using your initial moulds and a 3D computer system to create all the steps till your teeth will be perfectly aligned.
Since there is some time between the moulds and the moment you get your Invisalign it is possible for the trays to be a bit tight. I think this would best describe the feeling you have when putting the first aligner on or when changing with the next set. They are tight.
The entire system relies on the fact your teeth need to move to a desired direction. In order to achieve this, each aligner should push or rotate the tooth and it needs to not be a perfect fit for the current situation, but to force the tooth some more.
After wearing your trays for some time, you’ll notice the current set fits nicely. At the end of the 1 or 2 weeks / set you’ll be able to take out the trays very fast. The next set will be a bit tricky to put and after few days its getting easier again. By doing this, each set gets your teeth moving to the desired situation.
Getting back to the question at hand.
Its normal for the first aligner to not fit perfectly or appear to not be able to be put on your teeth. Normally your orthodontist is the one to put it in place, so he/shell be able to see if there are any issues. There shouldn’t be anything serious, just the fact its gotta be tight, as I mentioned before.
So, I don’t think you need to worry too much.
The pressure you feel when placing your aligners is normal and is actually vital for the entire orthodontic process to work.