If you close Synaptic by clicking the X in the upper right corner
(which may be something my window/desktop manager (LXDE) attached),
it's not the same as doing File->Quit. The lockfile doesn't get
removed, so apt commands don't work. I had to pkill synaptic, then
bring it back up and close it from the file menu, that removed the
lock file. This should happen on window close I think, no matter how
the window gets closed.