I've noticed this as well. Tried a scout/bowman/warsling sniper.

Skirmish works fine in melee, but the damage does not trigger at range even if I am within 30ft, and have moved 10ft in the round.

I've attacked from stealth, flanking, and getting a standard sneak attack (and the AC bonus from skirmish)  But the additional Skirmish damage isn't added or registered.

Please, if there is a fix that would make this class playable - as it stands it really spoils the class if its ability only functions in melee.