Daniel,
Welcome to the Lugerforum
Here is a blueprint image that shows the holdopen device installation location looking down at the grip frame with the upper assembly removed.
If you disassemble the top half of the gun from the grip frame, and look down into the grip, you will see the holdopen if it is present in this gun.
the holdopen device is operated by the magazine button when the magazine is empty... it pushed up the holdopen to catch the bolt when the last shot is fired or if the toggle is manually pulled all the way back with a functional empty magazine.
This function can be seen with the right grip removed.
Another reason that the holdopen might not work is that the inside of the wooden grip may be slightly swollen from moisture content or age, and it may impede the magazine button from reaching the full top position to operate the holdopen... If this turns out to be the reason, slighlty relieving the inside of the grip where it bears on the mag button will solve your problem.
Please post a photo of the inside top of the frame with the upper receiver removed and we can make a definitive opinion on the status of your holdopen.
All of the above is true only with original magazines or magazines that are made to original specifications. Triple-K aftermarket magazines for instance, have non-spec (smaller) mag buttons that will not reliably operate the holdopen.
...and now a word from our sponsor 
The image posted above was created from the set of Luger blueprints that I sell on CD... This month the CD is 20 percent off the regular price of $25.00 and is only $20.00 postpaid first class mail...
If you are interested in purchasing the CD, send me a private message or you can let me know in this thread, or my ad thread at the top of the "For Sale " forum...