The luck system in games should be additive, 

in your game it's not. Not every turn should be based on percentage for every turn from start. 

Your game: if 50/50% chance of getting something. 

1st try 50/50 chance, 

2nd try 50/50, 

third same and so on, 

that way i can try 10 times and still not get it.... 

How it SHOULD be: 

1st try 50/50 chance, 

but second try need to be added to the first chance, 

2nd try 75/25 chance, 

third 87.5/12.5 chance. 

So that you CAN get something after few fails.... 

The reason I give notice that is because I have been grinding Heiress (a stupid 3 star champ) for over 4 days, I should spend 3K energy on that and still did not get her, 4 days of wasting all my energy on that and still nothing....