Why doesn't form validation work on mobile?

Question

AJAX

Form validation does not want to work on mobile screen resolutions. Here is the code for form validation

Form validation code

(function( $ ){
  //// ---> Check issue element
  jQuery.fn.exists = function() {
     return jQuery(this).length;
  }
  $(function() {
    if(!is_mobile()){
      if( $('.phonefield').exists()){
        $('.phonefield').mask('[phone]');
      }
      if( $('.form_check').exists()){
        $('.form_check').each(function(){
          var form = $(this),
              btn = form.find('.btnsubmit');
          form.find('.rfield').addClass('empty_field').parents('.rline').append('<span class="rfield_error">Заполните это поле</span>');
          btn.addClass('disabled');
          // Функция проверки полей формы      
          function checkInput(){
            form.find('.rfield').each(function(){
              if($(this).hasClass('phonefield')){
                var pmc = $(this);
                if ( (pmc.val().indexOf("_") != -1) || pmc.val() === '' ) {
                  pmc.addClass('empty_field');
                } else {
                  pmc.removeClass('empty_field');
                }
              } else if($(this).hasClass('mailfield')) {
                var mailfield = $(this);
                var pattern = /^([a-z0-9_\.-])[email protected][a-z0-9-]+\.([a-z]{2,4}\.)?[a-z]{2,4}$/i;
                if(pattern.test(mailfield.val())){
                  mailfield.removeClass('empty_field');
                } else {
                  mailfield.addClass('empty_field');
                }
              } 
        else if($(this).is(':checkbox')) {
        var count = $(':checkbox:checked').length;
        var checkBox = $(this);
        if(count >= 1){
          //if(checkBox.is(':checked')){
            checkBox.removeClass('empty_field')
          //}
          }	
         else {
                  checkBox.addClass('empty_field')
                }
              }
        else if($(this).val() !== '') {
                $(this).removeClass('empty_field');
              } else {
                $(this).addClass('empty_field');
              }
            });
          }
          // Функция подсветки незаполненных полей
          function lightEmpty(){
            form.find('.empty_field').addClass('rf_error');
            form.find('.empty_field').parents('.rline').find('.rfield_error').css({'visibility':'visible'});
            setTimeout(function(){
              form.find('.empty_field').removeClass('rf_error');
              form.find('.empty_field').parents('.rline').find('.rfield_error').css({'visibility':'hidden'});
            },2000);
          }
          //  Полсекундная проверка
          setInterval(function(){
            checkInput();
            var sizeEmpty = form.find('.empty_field').length;
            if(sizeEmpty > 0){
              if(btn.hasClass('disabled')){
                return false
              } else {
                btn.addClass('disabled')
              }
            } else {
              btn.removeClass('disabled')
            }
          },500);
          //  Клик по кнопке
          btn.click(function(){
            if($(this).hasClass('disabled')){
              lightEmpty();
              return false
            } else {
              form.submit();
            }
          });
        });
      }
    }
  });
})( jQuery );

Not available on mobile

'<span class="rfield_error">Заполните это поле</span>

and an empty form is submitted