While most nibs will fit in the flange without needing to adjust it, some may slip. If the nib is slipping, it means that the curve of the nib exactly matches the curve of the flange. If it matches, there is no tension to hold the nib in place.
But, fear not! It's easy to adjust the flange if needed. Don't be afraid. Just push with your fingers. It won't look like you have even done anything, but it takes very little movement of the flange curve to create that tension that will hold your nib in place. Now, you will feel like a pro pen maker's assistant.