You haven't used enough gems to expect to get a 6*. 300 gems is 60 rolls (not counting any +1 bonus from rolling 10 at once). Since rolling a 6* is a half a percent chance, the mean expectancy to roll a 6* doesn't come until 200 rolls. Of course it's random, so some lucky people have gotten one or more 6*s at many rolls fewer than that, and some people will take way more than 200 rolls to get one. But nobody should begin to complain until they've spent at least a thousand gems without getting a 6*.