If you have used gradient shimming or another type of auto-shimming and you have peculiar line shapes, then you should check the following:
1. Was the depth set correctly on the tube?
2. Does the tube have a column of solution of approximately 5 cm? Tubes that are underfilled or overfilled will cause problems.
3. Is the sample well mixed? (see BLOG entry for September 4, 2007).
4. Is the solution cloudy?
5. Are there floaters or sinkers?
6. Is the sample paramagnetic?
7. Is there any slow chemical or dynamic exchange?