I have a Panasonic DMR E55 DVD recorder. I find that some discs that I
have recorded on it play back once and then not again. Or they play
for a bit on the Panasonic and then stick ( usually towards the end) -
 but will then work fine on my Hitachi DVD player - but then
frustratingly not work on the Hitachi if I try and play the disc
again. It's as if the act of playing a home recorded DVD disc can
change it in some way. The discs particularly don't like being skipped
forward. What is going on?  What are the properties of DVD's. Is the
problem the discs or the Panasonic?

Some players are very particular about the dye formulations and error
rates on recordable DVD blanks, both in the quality of writing and
subsequent reading. You don't say what type of blank you are using, it
may be worth trying several.
This recorder has mixed reports, both for general performance and
pickyness over disc selection.
http://www.videohelp.com/dvdrecorders.php?DVDname=Panasonic+DMR+E55&Submit=Search&hdsize=Any&orderby=Name&hits=25&Search=Search

I would check a few things. First make sure that you are using the
proper format for your DVD disks. There are DVDR- and DVDR+ disks.
Also make sure that you have the proper Video Format. NTSC is used in
noth america and PAL is used in other parts of the world. I would
check these things out and make sure that they are all consistent and
that your using the proper disks with your equipment.