in the last 2 season, I often tied a game in regular time and lost in overtime. My players are all very young and unexperienced and dont have full stamina -> is that the main reason?

The dice rolls are against you if your players have stamina below, say, 6 and you're going into the last minutes or overtime with the scores level. If the scores are level the teams are usually evenly matched, so the stamina may come in to play a little at that point, but it's mostly dice rolling.