Maybe.. just maybe..
You have no "immediate" relationship with them.
We have a very good job in an international firm, and people always ask us to write
an invitation to them. My husband never refuses anyone because he knows people don't just
get visa because of an invitation only. Some are students, our cleaning lady, gardeners,
handy men. Of course, we know them, and they work for us but we don't have "direct"
relationship. Can you imagine each summer they always ask us to write invitation for their
relatives?
Once in a while I send invitation for my university friends. They came to visit me. But that's
nothing to do with me at all. They have good job, stable financial prove.They could get a visa by themselves,
my letter just an extra piece of paper.
Try again with your own paper, not invitation.
I've never invited my family, they come on their own document. It's less complicated.
Good luck, next time!