True julii are incredibly hard to come across. It is more likely that you have Corydoras trilineatus, which are often sold as julii Corydoras. So much so that they are now referred to as the false julii cory. Not a name I like for them though, the three striped cory is much better.
Both trilineatus and sterbai Corydoras can be triggered by flowing water. It seems your sterbai just haven't gotten to that point yet. edit* and on Thursday you said you only have one sterbai, if that is the case that can explain why as well.
Both trilineatus and sterbai Corydoras can be triggered by flowing water. It seems your sterbai just haven't gotten to that point yet. edit* and on Thursday you said you only have one sterbai, if that is the case that can explain why as well.