Differing skill levels?

So in the past 3 days I've gone up for silver 4 about 5 times I think. Leading up to promos, my teams are great. We communicate well. We work together. And we win as a team. For some reason, when I go up to get promoted, I get stuck with someone who either has no clue what they are doing, decides that this is the game they will feed just because they don't like someone, or come down in a fight they were never in to begin with and we were not losing just to ult onto a surefire kill for the adc and basically ks. My theory that I'm putting together, is that you play with intelligent people who are willing to work together before promos, and then promos is basically "here's a bunch of idiots, carry them." Thoughts?