The amount of spin a planet starts with depends on how the momentum of the accretive material came together to create its angular momentum. Imagine a zone of the accretion disk of a star which comes together, like an ice skater pulling in his arms, thereby rotating much faster, and in the same direction as the disk. This can be increased or decreased by further off-center impacts of smaller bodies. Your planet may have had some major impacts early on at an angle that countered its rotation.
The spin slows gradually from tidal effects: motions and shifts in the planet from the variation of gravitational forces as it rotates in proximity to another celestial body. That energy comes from the rotation and reduces it. Clearly, the closer and more massive the other body, the greater the effect. Your planet may also be squishy or crumbly.
To truly lock, there must be some slight unevenness in the density or shape so that there is a tiny bit more pull at some points of rotation, so it should not be too spherical and should have lumps of differing density.