GROUP BY / ORDER BY DESC,(rand)

i have a problem, i hope that somebody can help. In code i have:

"SELECT * FROM app_generator_cviky WHERE partie = '$transformnazvu' AND stupen <= '$urovenkonkretnihocviku' AND (pohlavi = 'Obě' OR pohlavi = '$pohlavir') GROUP BY zarazeni ORDER BY poradi DESC, RAND() LIMIT $limitcvikuvpartii " - but its dont random. I always get the first one in group .... the rest of this is running ok.Whats wrong?:-) thx a lot

the db look like: db look like