angularjs model value is undefined in form

I have two forms(in one page). On first form there are two buttons NEXT and BACK. On clicking on the next i will render next form on page. I am using ng-if for this condition.

<form name="otpform" novalidate ng-if="renderotpform">
  <fieldset ng-disabled="otpdisable">
    <div class="middle-container steps fourth-step overflow-hidden" id="divotp">
      <input class="" type="text" name="otp" placeholder="{{ 'OTP' | translate }}" ng-model="otp" required >
      <input type="button" value="{{ 'BACK' | translate }}" class="brown-button" ng-click="gotoAck()">
      <input type="submit" value="{{ 'NEXT' | translate }}" class="blue-button" ng-click="checkotp()">
    </div>
  </fieldset>
</form>

Below is my js code.

 $scope.checkotp = function () {
                debugger;
                var otpvalue;
                $scope.otp = {};
                otpvalue = $scope.otp; //undefined
              }

When I try to access the otp model I am getting undefined property. On previous form I have next button. Inside next button I have $scope.renderotpform = true; so that above form is shown. May I know why I am not able to access OTP in the above code?