Extremely Low SR Gain

Im in bronze. I generally play main tank and main healer, if it matters. Today i won two 5v6 games (my team had 5 players) and won 2SR in both. Normally i get around 20-25, ive heard that SR gain depends on performance and teams' average SR but in both games the teams were close and i think i did well. Does anyone know why this is happening, and what can i do to avoid it?