trouple with sneat attack

[akata]

i play a rogue/fighter and last night i was out hunting orc´s with some friend and if i attacked first i didn´t get my sneak attack i know i dont get it if i dont beat them in ini. but even after the other fighter step up and i should get it for flanking i didn´t. Have anyone had the same problem?
and it was only if i attacked first

-Akata

[Thunder Pants]

if you are using a missle weapon you might be outside of the minimum range (think it's 30 feet for bows 40 feet for xbows, 20 feet for slings) if your using a sword it could be that the monsters is doing an action that doesn't have an enemy target and as such you arn't flanking them