We've had a few headaches with packing slips too. We were first using the shopify default, but then switched to Apps to be able to print them in bulk.
Currently we're using personal print for our invoices because it lets us smartly personalize our invoices for each customer, and they can then be downloaded directly from the Shopify orders' list page. It's incredibly the only App we've found with a drag and drop invoice builder that truly let us build what we wanted.
We're also using it to send some automated personalized thank you notes to our customers for about a year, and we love it for that!
I've been having issues with this randomly for a while. Sometimes, if I void the label and recreate a new one, the packing slip will reset and print correctly. However, today, I discovered this discussion board because I've been trying to print a packing slip for an order that I didn't even create a label for, I just entered the tracking number.. and the label that keeps popping up in a random label from an order about 3 weeks ago. This is clearly a shopify glitch, and I hope that they fix it soon. If anybody has any suggestions on how to fix this, please let me know!
There are many apps available to print the packing slips in bulk. In case you are using FedEx for shipping, use the Shopify FedEx app which will help you print shipping labels as well as packing slips in bulk for all your orders. Also, you can display FedEx rates and handle FedEx tracking as well.