SOLR has indexed too few products

Two days before I have noticed that search doesn't return a number of products . I found that the successfully finished indexing contained too few products. When I ran on-demand SOLR full indexing and later on the afternoon the index got corrected On the next day after a check of the amount of products in the index I found that there's again too few products. The planned resolution is to restore data from nightly backup and nightly indexing will be disabled.

Can Any one help me on this.