From what I understand, Clinton lost (despite winning the popular vote) because she took a couple of states or so for granted towards the end, and because a fair number of people assumed she would win and therefore did not feel compelled to participate in the vote. That won't likely happen again.
Oh, and the Russian interference thing.
There may have been a few other reasons as well but they were just rounding error, I would imagine.
And the fbi re-opening another dud investigation 11 days before election in 2016.
Hillary had a lot of outside opposition helping Trump and a couple of self inflicted wounds but today Trump is the incumbent and only 5 incumbents have failed to get re-elected.
It will be a close election and I mean that even within the context of most modern presidential elections are close.