My PHP upload script is not uploading all the multiple files, it seems to upload random ones. No errors appear, I can't find the fault

Question

I have a HTML form which I send to a php page, the form seems to upload some files but not others. For example if I upload six files, what seems to happen is 2 or 3 will upload and the others won't. I also send the filename to a database, which shows the same 2 or 3 files repeated.

I have checked all the timeout options, post sizes and cannot fathom the cause. I haven't placed any restrictions on the file types (yet).

The HTML code:

<form action="" method="POST" enctype="multipart/form-data">
    <label for="frm-activity-files"><h3>Attach files:</h3></label>
    <input type="file" multiple="multiple" name="frm-activity-files[]">
    <p>Files limited to 10MB each.</p>
</form>

The PHP Code:

<?php
$frm_upload_count = 0;
$frm_upload_path = $_SERVER['DOCUMENT_ROOT'].'/uploads/';

if (!empty($_FILES['frm-activity-files']['tmp_name'])) {
    $error = '';
    $frm_upload_count = count($_FILES['frm-activity-files']['tmp_name']);

    for ($i = 0; $i < $frm_upload_count; $i++) {

        $frm_upload_newname = (md5(date('Y-m-d H:i:s:u')).'-'.date('YmdHis'));
        $frm_upload_file = $_FILES['frm-activity-files']['name'][$i];
        $frm_upload_temp = $_FILES['frm-activity-files']['tmp_name'][$i];

        $frm_upload_ext = pathinfo($frm_upload_file, PATHINFO_EXTENSION);
        $frm_upload_rename = $frm_upload_newname.'.'.$frm_upload_ext;

        move_uploaded_file($frm_upload_temp, $frm_upload_path.$frm_upload_rename);

        $sql_insert_attachment = 'INSERT INTO data__attachments(attachment_auuid, attachment_uid, attachment_filename) VALUES (?,?,?)';
        $stmt_insert_attachment = $mysqli->prepare($sql_insert_attachment);
        $stmt_insert_attachment->bind_param("sss", $frm_uuid, $frm_uid, $frm_upload_rename);
        $stmt_insert_attachment->execute();

        $frm_upload_newname = "";
        $frm_upload_file = "";
        $frm_upload_temp = "";

        $frm_upload_ext = "";
        $frm_upload_rename = "";

        $error .= $_FILES['frm-activity-files']['error'][$i]; // get the error
    }
}