unable to move app into folder ios 12.4

I'm trying to move an app into a folder (in iOS 12.4), something I've done successfully many times before. Occasionally, though, something happens that is SO frustrating...when attempting to slide the app into the folder, the folder just moves away and will not let the app in. I have no idea what's triggering this or how to fix it. Any assistance would be much appreciated. Thanks!