ASM4: Can't use external geometry for sketching

[theber]

OS: Manjaro Linux (KDE//usr/share/xsessions/plasma)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.23578 (Git) AppImage
Build type: Release
Branch: master
Hash: 50c3cbf00579dc4941ca743c25720d016b0453ce
Python version: 3.8.6
Qt version: 5.12.5
Coin version: 4.0.0
OCC version: 7.4.0
Locale: English/United States (en_US)

I'm trying to make an assembly based on the methods used in Zolko's Tutorial 2, i. e. create the parts inside a single document. Maybe I'm doing this wrong?
I'm creating several bodies inside the first part so that I can fuse them with the PartDesign boolean tool. After I pad the first sketch I try to create a second sketch on top of the first pad. However I can't select external geometry to constrain the second sketch, neither from the first pad nor from a "Master sketch" made inside the Model. Am I trying to do too much at once?