Improve passing & scoring skills on attacking players through buying/selling and/or training

Improve defence by getting a better goalkeeper

Improve defence by training/buying defenders

Just bad luck

Hire as many doctors as you can afford (no more than 10)

Buy a similar player to replace him

Try to have a squad that is balanced

Make sure you have a default lineup set (in case of server crash or being unable/forgetting to log on)

Make sure that any injured/banned players are not in your lineup

Always set a provisional lineup several days before your next game-you can always 'tinker' a la Claudio Rainieri closer to the match

Look at your highest (normally solid) leadership players-if any are nasty or controversial they are probably affecting your Team Spirit adversely

Look at your coach's leadership-if he is poor or below he is not able to maintain your Team Spirit at high levels.

If you buy & sell a lot of players every week this is a side affect of that.