They cheat in different ways. They cheated against Sanders and Trump in the 2016 primaries by setting the machines to weigh the votes against them. This is shown by statistical trends that didn't exist in previous elections. They also voted against Trump in the general election in some cities using another method, but it wasn't enough to swing it. In 2016 they solved that statistical anomaly and focused their cheating in the swing states.

If they are way ahead they don't need to resort to these obvious cheating methods and can instead cheat in ways that are harder to detect.

They cheat in different ways. They cheated against Sanders and Trump in the 2016 primaries by setting the machines to weighing the votes against them. This is shown by statistical trends that didn't exist in previous elections. They also voted against Trump in the general election in some cities using another method, but it wasn't enough to swing it. In 2016 they solved that statistical anomaly and focused their cheating in the swing states.

If they are way ahead they don't need to resort to these obvious cheating methods and can instead cheat in ways that are harder to detect.