You can summon up to three Friends per day at the Hall of Heroes. To circumvent this restriction, build Amphoras in your City. Each Ambrosia Vessel will allow you to summon one extra Hero per day, free of charge. If you are unable to summon a Friend, this means that:
- You have already hit your daily limit (so either build another Ambrosia Vessel, or wait until the next day), or
- You have already summoned that Friend, and he is already within your group of Units.