After stitching in the 3D window, it's worth checking that the garment pieces and stitching are properly aligned. For example, you might have moved a pattern piece to make it easier to stitch. For the best simulation results, you need to return the pattern piece to its rightful place.
To check the garment:
On the Main toolbar, click Prepare.
Go to the 3D window, and check the pieces and stitching.
If a piece is too low, pull it up.
If a piece is too high, pull it down.
If a collar is not sitting correctly, adjust it left or right or forwards or backwards as appropriate.
If stitching is twisted, either turn the piece to untwist the stitching, or delete the stitch and do the stitch again.
Once you have checked the garment and made any necessary adjustments, continue with Dressing the Avatar.