Are you running in 64-bit or 32-bit? If you are running on an 64-bit OS, then
your problems are understandable as there is no 64-bit version of flash and you
need some help to get it working.

If you are running 32-bit Xubuntu your problem is not easy to explain. You need
restricted extras to match the version of Ubuntu. Don't search for restricted
codes, just restricted. They can found in the metapackages sections (there are
three of them) and they are specific, separate ones for Ubuntu, Kubuntu and
Xubuntu. You can install flash non-free separately if you want. You get more
with the restricted extras, Windows ttfs and multimedia codecs.

If you have installed Gnash, uninstall it by choosing Remove Completely in
Synaptic. You might want to remove other things that you installed and start
over.

If you are running the 64-bit version of Xubuntu then multimedia won't work
properly since there aren't any 64-bit versions (don't blame Linux since it is
the product of Adobe and they have not been forthcoming). With some fudging you
can get it to work, but this is not satisfactory for most people. You will find
other problems as well, with Real Audio and Quicktime. If you use multimedia a
lot then install the 32-bit version of Linux (any distro) on your 64-bit
computer. What you lose in speed you will gain in lack of frustrations.