Exactly this... only luck and a big (huge) bankroll can help in winning on slots. Crazy long losing streaks and bonuses that don't pay anything are completely normal things while we play slots, so to hit something big and meaningful we really need a big (huge) luck. As a regular slot player, I had many crazy big wins, but I also experienced "dry weeks" when I couldn't win anything, losing deposit after deposit without any significant win.

So yes, luck is the main factor here, without luck, we can't do anything in slots except watch our bankrolls going down to zero...