Sonarr isn’t designed to actively search for anything, old or new, under normal operations (exceptions are mentioned in the first FAQ link below).
Whatever passes by on your indexer(s) RSS feed and matches a wanted episode in the right quality, correct restricted words, size, etc will be grabbed.
Old things may be reposted and appear again on the RSS feed and subsequently grabbed by sonarr, but when that will be is anyone’s guess.
This is covered in the FAQ, albeit spread out a bit, how does sonarr find episodes and somewhat related why doesn’t sonarr automatically search for missing episodes.
TL;DR: if you want old stuff now, then yes, perform a manual search.