My type 4 Radom has a slide lock issue too.
If hand-racking the slide, it locks back with a mag in.
If the mag is ejected, it will unlock the slide.
If no mag is in and the slide manually locked back, it will self-unlock after a half second or so.
When shooting, it won't lock back on empty 70% of the time as apparently the recoil is severe enough that it overcomes the follower spring.
I came to the conclusion that the slide catch slot wasn't cut square enough (like kman).
You could be right ...
I've read somewhere that the "type 4" Radoms were produced towards the end of the war when the Germans tried to speed up production by omitting things like the take down latch and that the overall quality was pretty crude.