要解决ASP.NET MVC 5中远程验证不起作用的问题，可以按照以下步骤进行操作：

1. 确保在 View 中正确设置了远程验证的属性，并指定了远程验证的 URL。

@using (Html.BeginForm())
{
    @Html.AntiForgeryToken()

    

        @Html.LabelFor(model => model.Email, new { @class = "control-label" })
        @Html.TextBoxFor(model => model.Email, new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.Email)
    

    

        @Html.LabelFor(model => model.Username, new { @class = "control-label" })
        @Html.TextBoxFor(model => model.Username, new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.Username)
    

    

        @Html.LabelFor(model => model.Password, new { @class = "control-label" })
        @Html.PasswordFor(model => model.Password, new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.Password)
    

    

        @Html.LabelFor(model => model.ConfirmPassword, new { @class = "control-label" })
        @Html.PasswordFor(model => model.ConfirmPassword, new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.ConfirmPassword)
    

    

        
    
}

2. 在服务器端，确保远程验证的方法的签名正确，并且返回一个 JSON 结果。

[HttpPost]
[AllowAnonymous]
[ValidateAntiForgeryToken]
public async Task IsUsernameAvailable(string username)
{
    var user = await UserManager.FindByNameAsync(username);
    bool isAvailable = (user == null);
    
    return Json(isAvailable, JsonRequestBehavior.AllowGet);
}

3. 确保在客户端正确地引用了 jQuery 库和 jQuery Unobtrusive Ajax 库。

4. 确保在客户端正确地初始化 jQuery Unobtrusive Ajax。

使用上述方法，可以解决ASP.NET MVC 5中远程验证不起作用的问题。