The one of the worst parts about leaving a project alone for too long is losing the voice you had for the characters.
When I'm constantly working on a project, the voice just seems to flow. All the prose fits together and it feels like it all belongs to the same story. Or at least it's not too disjointed.
But sometimes, when I leave off for a few days or a few weeks and come back, it takes time to get back with the voice of the story. I have to read over what I've already written and try to continue from there. But even then, there is often a lag between where I left off and where the voice comes back correctly.
It also doesn't help if I read something with a very distinctive voice in the interlude between writing. It tends to affect my voice when I pick my writing back up.