It's entirely possible that you are simply experiencing the completely natural vague and unpredictable nature of the kind of random selection system which is used to determine what is in Sticker Pack that you purchase.
The best simple analogy would be to think about flipping a coin; there is a natural assumption that with odds being 50/50 if you flip a coin 100 times you'll get 50 Heads and 50 tails but the reality is often radically different from that and you could wind up with long runs of just getting Heads or just Tails and wind up with results skewed heavily towards one or the other.
The Sticker Packs work on a similar principle and when it comes to Legendary characters, which are the hardest to get, some players get "streaks" where they get a lot of character pieces in a short time or the opposite streak where they get nothing.
The most recent update, Frontline Fighters, introduced Nec'Rose, Commando Corn, Scallywag Imp and Captain Squawk and when I started buying the packs to get those characters I received all of the pieces to every character except for Commando Corn after spending less than a million PvZ coins but then had to spend something like four or five million more coins before I finally completed that last character.
