I think Pennywise was paying the Losers constant visits
I get the impression that Pennywise paid more than the few visits to the Losers and other children of Derry that we see in the movie, and indeed scared them most nights. For example, Stan mentions the "woman that I keep seeing", or rather, that painted thing that we see with him when it possibly first appears and scares the living daylights out of him. It may have come back in the middle of the night, as a shadowy figure at the foot of his bed or something, grinning.
Of course, the downside of that is that the scares would no longer be scary after a while, but Pennywise, I'm sure, took on other forms to maintain the scare factor. And the Losers, not knowing fully what was going on yet, would still be scared.