Apps hidden behind Plasma on Lubuntu-Qt

Lubuntu-Qt 18.04 is fast & very attractive on a 2.66 Ghz Intel Core2 CPU box with 4 gb of ram & NVidia GeForce GT 610 video card. Congratulations to the developers. I later installed the Plasma desktop. When I log out of Lxqt & into Plasma, I only the desktop, menu bar and main menu are visible. All running apps, config menus & open documents are hidden. Only the terminal appears with Alt-F2. This problem does not appear on a Celeron laptop with Lubuntu- 18.10 + Plasma. Any suggestions?

LXQt is not supported in 18.04, neither by Lubuntu nor Ubuntu. Please use 18.10 if you would like support.

This site is for the co-ordination of the Ubuntu project(s), and not support. For Ubuntu Support options, please have a look at (though I would take note of Simon’s already provided reply)