Sync PouchDB with server

I'm currently working for a startup which ask me to develop an offline app using pouchDB. This pouchdb is used to store the data entered by users.
The offline application works fine. Now I have to add one feature on the online app to sync the dbs. After a login, the online app have to check if data is stored in a pouchdb on the device which is connecting, and, if the check found data, the online app have to pull this data.
I have the folowing problem: the online app can't get the db stored localy on the device (even if I run the both app in the same browser).

I explain my problem in another StackOverflow post, but the formulation was not so good so I think that's a good thing to post another question.

My old post here

I work on this problem during few days and I don't have much time until I have to finish my work, if someone know how to solve this, it could be very nice :)