No new character stickers

What you are experiencing is the normal variation in how statistical probability works. Some players will experience good luck with the system and will collect character stickers at a decent rate and others will experience bad luck with the system and see a much lower drop rate for them.

It's not a bug and there's nothing which can be "fixed". The sad truth is that RNG based award systems are notoriously unreliable on a small scale, like one player opening a small number of packs, and only look "good" when dealing with huge sample sizes, like looking at the drop rate for the community as a whole with hundreds of thousands or millions of packs opened.

All you can do is continue to collect your coins and purchase your packs until you get your characters, however long or short a time that might take.

I got super lucky with my characters but my wife on the other hand spent 8 million and did not get either legendary character and got her regular ones after spending almost 6 million.  Its a turkey shoot whether you get one or not good luck and keep on playing.