Come September/October, the incumbent will rise at least slightly, sometimes significantly.
Go back and look from Ronald Reagan to Barack Obama. Only one president did not.
The last one before him to suffer the same fate? Jimmy Carter. Also, economic underperformance, or "malaise".
Truly persuadable voters will ultimately ask...
"Do I really want to take the risk of rocking the boat? Things are going well. I don't think it's worth it..."
If economy is solid and no war, then most pull the lever for an incumbent.