Hextech chests

Looking at your match history through op.gg, I see that you pretty much only play Vayne, with a Blitzcrank game every once in a while. You can only earn one hextech chest per champion per season, so once you earn a chest with a champion, you will be unable to earn anymore chests with that champion until next season. If you want more chests then you will need play some different champions.

As ZombieMaster said, you can only get awarded one chest per champion, per season.

If you want to see if you have already been awarded a chest, there is an indicator in the champion collection tab.

https://support.riotgames.com/hc/en-us/article_attachments/203324076/Chest-Earned.jpg

In this example, the boarder around Anivia's icon indicates that the player can no longer receive a hextech chest with that champion for the remainder of the season. You can also see the rank achieved to earn the chest in the pop-up.