copy/paste problem

[rikona]

Recent problem which seems to be getting worse. Running Ubuntu 16.04
updated. I have 2 'files' windows open, highlight some files in #1,
release all mouse buttons, click on the highlighted files, hold down
shift key, drag to #2 window - and nothing happens. Repeat, repeat some
more and finally works on perhaps try #6-10 - finally moved the files.
Getting to be a pain. What might be going on and how can I restore the
original move/copy behavior? I have 15-20 'files' windows running and
usually select 2-4 of them on the screen to work with. I also have many
other pgms running as well. Is this perhaps an overload problem? If so,
how can I check that to keep below 'overload' and/or identify the pgms
most responsible?

Rik