What you are experiencing is the built-in uncertainty of using a Random Number Generator to award Rare, Super-Rare and Legendary items.
A chance of receiving a character piece is just that- a chance and not a guarantee.In addition that chance become smaller with the rarity of the character in question; Legendary characters can be notoriously difficult to acquire.
With the way that RNG systems work some players can quickly be awarded character pieces while other players can spend millions in PvZ Coins and still come up empty. It's not a buggy system, it's just unfortunately the way these sorts of random award systems work- how much you spend or how many times you haven't received something has no effect on the content of the next Sticker Pack that you open.
