Hostname is set to the device's serial number, so it doesn't change after
hard reset.
I think replication server considers the database as a new subscriber after
the device has been hard reset. Replication monitor shows a new subcription
(HostName + a unique id)  when I synchronize the database. The problem is
that my replication doesn't allow multiple subsciption per partition so this
synchro fails.

See MSDN :
"You can create the initial subscription on one smart device and then deploy
the application by copying the application and the initial subscription to
as many devices as you want. When the application first synchronizes, SQL
Server Compact Edition automatically recognizes it as a new Subscriber, due
to the new device and/or physical folder location change, and creates a new
subscription. By copying the initial subscription to a device, you avoid
downloading the initial subscription to the device over the network."
(http://msdn2.microsoft.com/en-us/library/ms172912.aspx)

My understanding is that SQL Server considers the device as a new one after
hard reset.

One more thing : I noticed that deleting the concerned partition on server
side (replication properties>partitions) seems to solve the problem, next
database sync are successful. However this is not a reliable solution as
devices are regularly hard reset due to battery fault.

Thanks for help !