Currently you get 6 new squad leaders in the introduction event of the strategie battle system. More are not available yet, but I'll add up to 20 squad leaders in total over time.
Were they defeated in battle and did not spawn again in the next battle? Do you have the most recent versio 20.07.19?
Else, you can also select what squads should go or not go into battle on the overview screen of the vanguard forces. Maybe you de-selected them on that screen and thus they don't come into battle anymore.
You can get the newest version for all systems on the main page at https://ryensaotome.itch.io/renryuu , or if you prefer to download somewhere else you can also get the game at https://mega.nz/#F!CpQXwBDS!wwZlWRrPWoGzZX5h0-MlRQ
The newest version is 20.07.19