Stand Alone One to One CD Copier Will not Start

In Dec, 2002, I bought a 40x CD duplicator on eBay, which can copy CDs
in most formats. Now I have a problem for which I will like to ask
your kind help.

The duplicator has worked excellently until recently, when it would
more and more often fail to (automaticly) start copying after both
the master and blank disk have been put in the drives. In the
beginning, when it failed to start, switching the unit on/off or
poping out the disk(s) and reloading it(them) might do the trick. Now
it will totally refuse to start with certain batch of CD-Rs (which it
worked with fine before, but now even above-mentioned trick will not
work. The blank disk will just stay in the drive. No copying will
start), while it will work with a different batch (though above-
mentioned trick is needed in some case). With CD-RWs, it will mostly
work. When it refuses to start, the LCD display will remain at "CH".
If you put a CD-ROM or a written CD-R (both are un-writable) into the
target drive, the disk
will not be spit out(the target drive is supposed to spit out any
disk which is unwritable).

Would you please kindly advise what's wrong with the machine if
possible.

Best regard.