I've printed a couple dozen items on my Anycubic Kossel Plus successfully. Today I tried printing a file and the extruder started laying down filament about 2" above the bed. The Cura model looked like it was sitting on the bed. I rebooted and tried a file I'd done before and it printed normally. Both files were sliced with Cura 4.2.1. What can I check to make sure the file begins on the bed?