That is strange indeed, and this same complaint about Homegroup has come up many times already here with no clear resolution to the problem.
Zigzag says that Homegroups is the worst thing ever but for me it's been El' perfecto so go figure.
For me, It's the best thing ever when it's working right and I've never had a problem with it.
The set up with networked machines was truely impressive for me, machine goes online after installing Op system first boot, prompt to join homegroups appears, type in the password,..done everything shared.
If that isn't how it happend for you then something is wrong.
I can easily stream movies, music files what ever to any machine. I tested a cross stream of movies, sharing DVD drives from one machine to the other back and forth between 4 machines and 4 different monitors and every movie played perfectly.
It's a confusing problem to be certain. I guess that all we can hope for is a patch or driver update.