I've won 5 consecutive games, why isn't my MMR being modified?

The number of wins/losses does not matter. MMR is a relative calculation that may be modified from improvement in algorithm. It can be either rise or fall. :)

Indeed, your MMR will increase if you do win a lot, yet it does not mean that it will rise instantly. It gradually increases as you win and lose matches, so take your time and try your best.