Twilight Struggle, Longer game
1 in 5 getting to the Late War isn't much of a sample size, but it does appear to be on the low side. Do you have more details on why it ended early? DEFCON, Europe Control or 20 VPs? Is it USSR or USA that's winning early? Is one of you always winning early?

I found this spreadsheet from this link depicting stats for a pair of players, and they get to the Late War about half the time. My own experience is 17 of 44 games decided in the Late War, which is on the low side because my opponents were newer and overmatched (I was 36-8). Games reach the Late War more often if both players are evenly matched.