Hi @agu, Jami is still a work in progress. It works very well for some people but not so much for others.
Also submitting bug reports is one way to help improve things 
Keep in mind that this is a very ambitious project. We’re trying to build a reliable communication platform that doesn’t depend on central servers. Considering the difficulty of this problem, it’s amazing how far the devs have gotten!
Anecdotally, Jami works very well for me on Linux, iOS, and Android, using wifi. I can’t always get calls to work when I’m on 4G LTE, though.
Also, there is a known bug where messages are sent but the client doesn’t update to display this, so this could be affecting you.