Laravel relationship does not load

Here is my Model class with defined relationships. And all of them work besides tax_class(). I have tried to debug many different ways but it just does not work, while others designed exactly the same way do... Is this a known issue and what could be the resolution. when I try to call the relationship it always returns null.

class Client extends Model
{
    use HasFactory;

    protected $fillable = [
        'manager_id',
        'email',
        'name',
        'type',
        'tax_class_id',
        'state_id',
        'city',
        'contract_type',
    ];

    protected $casts = [
        'tax_class' => TaxClasses::class,
    ];

    public function manager()
    {
        return $this->belongsTo(User::class, 'manager_id')->withDefault(); // Returns a default manager if none is assigned
    }

    public function tax_class()
    {
        return $this->belongsTo(TaxClass::class, 'tax_class_id');
    }

    public function documents()
    {
        return $this->hasMany(Document::class);
    }

    public function state()
    {
        return $this->belongsTo(State::class);
    }

    public function schedules()
    {
        return $this->belongsToMany(Schedule::class, 'schedule_client');
    }

    public function taxes()
    {
        return $this->belongsToMany(Tax::class, 'client_tax');
    }
}

and here is the TaxClass:

class TaxClass extends Model
{
    protected $table = 'tax_classes';
    use HasFactory;

    protected $fillable = ['name'];

    protected $casts = [
        'name' => TaxClasses::class
    ];

    public function clients()
    {
        return $this->hasMany(Client::class);
    }
}

Solution

Possible Issues

Method Naming Conflict

Laravel uses camelCase for relationships. If you name your function with an underscore (_), Laravel might not recognize it properly.
Example of a problematic relation:

Blockquote

public function user_profile() {
       return $this->hasOne(UserProfile::class); 
  }

Laravel might not detect user_profile correctly. Eloquent Tries to Guess Foreign Key Name

Laravel assumes the foreign key is user_profile_id instead of user_id (if using a custom table).

Solution: Rename the Function Using CamelCase

Instead of using an underscore, use camelCase:

public function userProfile() {

return $this->hasOne(UserProfile::class);

}