Make sure that you dont have noindex in html tags.
Another reason which is my opinion.Writing too much tags can be reason for this.Because if if you have many tags , that can be cause of many tags which are redirected to same page.
For example , you have a one question that is poor by content has more than 10 tags.So when you view each tag you will see this poor content.So think about 10 question with poor contents will have 10x10 =100 tags.I just gave this example because I sometimes I had faced this issue.