Yes affirmations are quite powerful in assisting you to manifest what you desire but not everyone is successful with applying affirmations.
Reason Number 1
If you are repeating affirmations on a surface level without giving them deeper thoughts and feeling then you will find that your results are quite slow. You may even want to give up doing the affirmations all together, which is quite common.
Reason Number 2
Affirmations take time to work. Depending on how deeply ingrained your belief is it may take a great deal of time for the new affirmations to replace the old thought pattern. Too many people give up a bit too soon before they are able to see results. Repeating affirmations should be fun. You should feel enthusiastic about your goal, so enthusiastic that you are willing to repeat the affirmations as long as it takes.
Reason Number 3
Lack of enthusiasm can block the flow your success. It's pointless repeating affirmations for a goal that you are not excited about. When you are enthusiastic about your goal your vibration will be high. Good feelings feed you a large supply of energy. The higher your energy the more impact your affirmations will have. Think of how you feel when you are dancing or have just had a good run. You may recall feeling powerful and enthusiastic about life. That feeling is quite effective in assisting your affirmations to move to deeper levels of your mind. This is one of a few powerful techniques that will enhance the effectiveness of your affirmations.