Various problems with the user-sync plugin

I have many problems with user-sync plugin:
- Some sites refuses to sync (when the initial connection was apparentely good - but without synchronization)
- some sites that appear in sync (synchronizing launch from the master) are not well synchronized... For example, a user should be an administrator, he finds himself subscriber ...

The logs do not provide any information ...
[2015-08-12 1:20:19 p.m.] [S] 0-10 - ajax shares ***

In short, nothing works and it bothers me a little, I spent two days on this problem by trying different installations (including all fresh installation).

Some help would be appreciate.
Thank you and sorry for my english...