Issue downloading some files using PHP curl - php

Question

I have a script that downloads PDF files after logging into another site. It has so far worked great for all sites but I am now getting something strange with a new site I'm scraping: some of the files downloaded are 1kb (i.e it didn't work) while others work just fine. Using the download link in the browser opens the "do you want to save this file" window and the file is correct there.
here is my code (I include both the general curl parameters used throughout the scrape, and the final part where I try downloading the files):
//Initial connection to login page
$header[] = 'Host: www.domain.com';
$header[] = 'Accept=text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8';
$header[] = 'Accept-Language: en-US,en;q=0.5';
$header[] = 'Connection: keep-alive';
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://www.domain.com/login');
curl_setopt($ch, CURLOPT_REFERER, 'https://www.domain.com');
curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Firefox/24.0');
curl_setopt($ch, CURLOPT_HTTPHEADER, $header);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
curl_setopt($ch, CURLOPT_MAXREDIRS, 10);
curl_setopt($ch, CURLOPT_COOKIEFILE, $cookieLocation);
curl_setopt($ch, CURLOPT_COOKIEJAR, $cookieLocation);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false);
$webpage = curl_exec($ch);
//Then several operations to login, grab the list of links to PDF download files (...)
//Loop through the array containing the url of the file to download and save it to a folder (writable)
curl_setopt($ch, CURLOPT_POST, false);
foreach($foundBills as $key => $bill)
{
curl_setopt($ch, CURLOPT_URL, $bill['url']);
curl_setopt($ch, CURLOPT_BINARYTRANSFER, 1);
$pdfFile = curl_exec($ch);
$randomFileName = rand_string(20); //generates a 20 char long random string
$newPDF = $userBillsRoot.$randomFileName.'.pdf';
write_file($newPDF, $pdfFile, 'wb'); //using a Codeigniter function to save the file
}
The files are under 1mb each. Any ideas? How can I see more details about why it's not working (e.g timeout)? Thanks!