Please use bilateral force while inserting the aligner; insert the aligner anteriorly and then posteriorly with bilateral equal pressure. This means please put the aligner onto your front teeth, and then using both the left and right fingers, press the aligners onto your back teeth at the same time, using outward pressure (if the aligners seem too narrow) or inward pressure (if the aligners seem too wide) to gently force the aligners on. It’s important to push both sides of the aligners onto the back teeth at the same time.
If that doesn’t work on its own, it will help to use cotton or foam chewies. You can purchase these online or at some pharmacies.