There can be multiple answers to this problem. The most common answer is the ad is using a picture which has too much text. Facebook has a rule where if a picture has over 20% text in the image you will not be able to use it in an ad. Pictures that don’t quite have 20% text but still have a lot of text can run into problems resulting in spending more money to reach your objective (post engagement, page likes, conversions, etc). A way to prevent this from happening is to use this website to check your images while designing them:

Another reason this could be happening is because of a ‘spammy’ headline in your ad set. If you ever name an ad set along the lines of: ‘FREE SEMINAR, GUARANTEED TO MAKE MILLIONS!!’ it will be more expensive to get what you want (post engagement, page likes, conversions, etc).

If your post gets unapproved entirely, your ad violated Facebook's terms and conditions and will tell you what you violated so you don’t make the same mistake again. However there are instances where Facebook can mess up and unapprove an ad when it really wasn’t violating anything. In that circumstance you can appeal it. Facebook will double check the ad to see if it broke any rules. If Facebook clears it, the ad will be approved and run like normal.