Can't buy items at random intervals (and frequent stutters)

Straight to the point, I'm a support main, mostly mages. Every other game I am buying items, the ones I can afford are highlighted, and I buy it. But at that point, the item will remain highlighted, and for the rest of the game I will not be able to buy anything that costed more than that, even if I have the correct amount. By which, I mean that they will remain dark, and not highlighted, and I will not be allowed to buy it. Ex: I will have 2 but will be unavailable for purchase, and will be labeled at 3600, and I have 5000, but still can't purchase it. When I'm 40 minutes into the game, and I STILL can't purchase anything above 1300, It is very inconvenient. I've lost four ranked games because of this crap.