Differences?

That's a pity! But don't give up, use their mailing lists, forums or whatever public facing services they use. Figure out why they don't want to co-operate. But also spend time letting them know you as well, what your skills are and give good reasons why they should consider your changes, be visible to the whole community. Getting accepted into a community doesn't happen instantly, you need to be persistent and patient.

Of course, if you're always rejected in an inappropriate fashion (esp. if it happens publicly), then it is a completely different issue. Then promoting your own fork instead of the upstream is fair enough. But please let that be the last resort.