On your Find Document Types page (Find > Overview > Document Types) you may have a large number indexed documents, possibly more than your index allow. When your index reaches its max number of documents it will index no more documents than the limit, so reducing the document count may be a priority.
Two very common types responsible for a large percentage of the document count are .Variation and .Commerce.FindSearchProvider documents which are being indexed by Find. You may not need these types indexed and you may need to save room due to index document number restrictions.
In the above screenshot(mostly blanked out for confidentiality reasons) you can see that there are a large number of .Variation and .FindSearchProvider document types.
.FindSearchProvider documents may also be removed. You do not have to index Commerce Manager with Find. In order that the commerce manager site documents do not count against your find index limit, you can use a different search provider.
Finally, if you have any non-essential languages you could greatly reduce the index count by removing those languages.