There is a known problem on Xorg with situations when some application prevents locking screen; it was described as unfixable in Bug 383327. Some related work was done here: https://phabricator.kde.org/D20804 The number of screen locker attempts was increased to 3, also an "AboutToLock" signal was introduced offering a possibility for 3rd-party apps to change their behaviour right before screen lock attempt. Plasma could launch a notification when screen locker fails due to Xorg problems. At least it will serve as an explanation for a user who will get a basic understanding of why a DE's security feature is malfunctioning. Text variant: "Some application prevented screen locking". A better text label might include an application's name, but I don't know if this is technically possible. Currently log doesn't get further than "Could not establish screen lock". Knowing an app's name will make some workarounds possible, also in this case users might show it to developers and, who knows, maybe somebody will implement "AboutToLock" monitoring in their code.