Why haven't I received any new recruits?

Do you mean the heroes from recruitment tickets. These are awarded randomly and unfortunately with a low change of getting the good ones. There are only a limited number of each type so you will get a lot of duplicates. The high level ones are rare and can take dozens or hundreds of tickets to gain. Some heroes are very very rare or don't seem to the recruited this way at all. For these you need to collect fragments with hero tokens and wait for them to be in the token shop, or pay money to get.