You should contact the seller who sold you the 
cels.  Hopefully, they can inquire with their 
local post office for you and help you find out 
if the package was lost in Japan or en route.  
Usually, there is more detailed tracking 
information from the sending country compared to 
the destination country.  Maybe you can find out 
some answers there.

More importantly, the seller is the person who 
will have to file for an insurance claim and try 
to get your money back to you.  So you should 
contact the seller immediately and tell them 
what happened.  If they cannot find out more 
information for you, it is the seller's 
responsibility to recover your money for you 
(even though it may take a very long time, you 
should get your money back).  Please remember 
that all EMS packages are insured.

Also, you should visit your local post office 
and provide them the tracking number.  
Hopefully, someone can help you find out more 
detailed information about the package.  And if 
you need to file a claim for a lost EMS package, 
you should be able to do so at that post office.