Suddenly can't edit individual notes?

I'm sure this isn't a bug but simply my own lack of experience with this software. I'm an experienced user of Sibelius from many years back so I've been able to figure out quite a bit in just a few days. Suddenly, after doing a bit of copying and pasting entire measures of drum tracks, I can't select a single beat anymore. Using mouse selection will only select the first beat of any measure. Arrow keys skip everything but the first beat as well.

What did I trigger and how do I get back to editing individual beats? Thanks in advance!