Manual focusing problem

I have a Panasonic Lumix FZ15. It generally does a great job of
auto-focusing, but sometimes I need to use manual focus. I find that at
least half of my manually-focused pictures are not sharp, even though
the focus looks right on in the viewfinder using the focus-assist
feature. I can't figure it out. Has any one else noticed this problem
with their Panasonic digital?

Duke