In recent browser versions, the window opened by the packing slip module to print your order contents would close before the print dialog would open. That’s been fixed in v1.1.1.
If you use a custom packingslip/standard.twig template, you’ll need to incorporate the fix manually.
Change:
<body onload="if (window.opener) { window.print(); window.close();">
to:
<body onload="if (window.opener) { window.print(); setTimeout(window.close, 100); }">
The similar-but-different third-party extension PrintOrder also requires a similar fix, which is already on GitHub.