Question About Indexing

Hi guys, I was hoping some of you could help me with some insight. So in the last 6 months I have purchased 2 decent expired domains. Nothing spammy and were legit sites before. The sites I built are niche related. Both site currently in sandbox. My question is in webmaster tools I see sometimes my indexed pages flux so like one week its 100 indexed pages the next week its 80 or something similar across both sites. My questions is that with proper seo/link building will those pages be readded to the index. As the site gets more trust and authority will pages that were initially not indexed be added just due to seo and time. Or will I need to go back and like edit those pages with more content / analyze why they are not indexed? Thanks for the replys