Whether Servants are locked depends on their rarity and whether you have summoned them in the past.
▼Rarity ★4 (SR) and lower:
The first time you summon the Servant, they will automatically be locked.
* If you have summoned them even once in the past, the automatic lock will not go into effect.
▼Rarity ★5 (SSR):
Regardless of whether you have summoned them before (even if it is your second or further copy of the same Servant), they will always be automatically locked.