AngularJS to php to json working inconsistently - php

Question

I used AngularJS to make an app for grading students. It works perfectly 90% of the time, but every now and then, it doesn't record the data. The grade given or other change made will be reflected in the DOM, but disappears when the page is refreshed.
I use an angular service to send data to a PHP file which sends it to a JSON file.
The service is set up like this:
app.factory('studentList', function($http) {
var studentService = {};
studentService.getInfo = function() {
var promise = $http.get('../data/studentList.json').then(function(response) {
return response.data;
});
return promise;
};
studentService.sendData = function(data) {
$http.post('../data/studentData.php', data)
.success(function (data, status, headers, config) {
console.log(status);
})
.error(function (data, status, headers, config) {
console.log(status);
});
};
return studentService;
});
When I give a grade or change student info, I call the sendData() function defined in the service. This sends it to the PHP file which looks like this:
<?php
$data = file_get_contents("php://input");
$file = "studentList.json";
file_put_contents($file, $data);
?>
I've used this PHP code before, and it seems to work fine in everything else I've done. Any insights?
Thank you.