Item description not showing up on details - paypal

Question

I have a cart form that submits to paypal with the item_name and item_number. As of recently though, some of the paypal receipts (transaction details) I have been receiving are excluding the "item_number" in the description. I need this number to know what products the user has purchased.
The only common factor with the receipts that do not include the "item_number" are ones that have a "Receipt No:" with the text "Please keep this number for future reference, as your customer doesn't have a PayPal Transaction ID for this payment."
Any idea why this is happening, or what I need to do to get the "item_number" to show up on the receipt?
Code Example:
<form action="https://www.paypal.com/cgi-bin/webscr" method="post">
<label for="first_name">First Name*:</label>
<input type="text" name="first_name" id="first_name" class="required">
<label for="last_name">Last Name*:</label>
<input type="text" name="last_name" id="last_name" class="required">
<label for="email">Email*:</label>
<input type="text" name="email" id="email" class="required email">
<input type="hidden" name="item_name" value="Personalized Frame">
<input type="hidden" name="item_number" value="|S5|M8|T8|black">
<input type="submit" value="Add to Cart">
</form>