There is a trick to this, I played 16 ranked games this season, I got to mid in 12 out of 16 games, all solo q. Sometime even if I am last pick, if you ask nicely, people will more likely to help since they wanted to have to best team possible.
By calling out "mid", you basically commanding 4 other stranger what they not allow to do, people don't like that, and some will even lock the position you want just to show that you are not their boss
Instead, tell them what you can't do.
"I can't adc or jungle, but I am best at mid", now you increased your chance to get the pick you want from 1/5 to 1/3. Everyone wanted to win, so naturally they will be more willing to give you mid, and you did this without sounding like you boss people around.
If someone asked you to go somewhere you didn't want to go, instead of saying "no", just tell them "yeah i will try, I haven't played xxx for a few months tho". Usually this will freak people out and yield you whatever role you called.
Don't worry about support, for some reason there are more and more support players around, I never been to a ranked game without 1 or 2 people screaming "support only!". So in theory by calling out the 2 roles you can't do, the chance of you getting the role you want is 1/2.