[crimar30]

Just purchased a new g19, haven't even fired it yet, and the slide lock won't engage unless there's a magazine in place, anyone have any idea why?

[JR]

The gun is doing what it is supposed to do.
If an empty magazine is in place the follower will lift the slide lock upward and the arresting notch of the slide will catch it.
If the magazine is not in place there is nothing to push the slide lock upward unless you manually do it.