Scrollback going away in lxterminal

One lxterminal window with several tabs has stopped saving a scrollback
buffer.  The only control I can find is Edit > Preferences > Scrollback
which says I have 880 lines of scrollback, but the thumb fills the slider
and rolling the mouse wheel works like the up-arrow key, displaying
previous commands.

This happend more or less out of the blue, but I wasn't really paying
attention.  The tab displays an ssh connection to a FreeBSD-current 
host, if that matters.

If anybody has a suggestion for turning scrollback on again please post!

Thanks for reading,

bob prohaska