Hope this helps someone to diagnose what is going on here. The brushstrokes with a red horizontal are the ones that won’t flatten. The ones in black DO flatten. I follow the same procedure, some flat, some don’t. Some are variable width, some not.
I can convert to brush strokes, then they flatten - but the feature that’s absent then is the way pencil strokes can be made to smoothen into each other as they draw…
I did install/uninstall. And it just doesn’t work.
It could be the wacom digitizer that causes the issue. I tried flattening lines with no wacom driver installed, and it also did not work. No errors in the message log. (I use Surface Pro)
I already gave up on getting it working correctly. I just use line connect (the one with hammer icon I think) as much as I can if I need a merged lines. So far it works nice but some crossed lines might end up disconnected from the rest. It’s a pain but… looks like that’s the only choice left.
Final fix to this problem (one that doesn’t involve a workaround) - I was on the phone with TB tech support, finally figured it out:
It was a Wacom/cintiq issue - solution was to clear the Wacom preferences file - it had become corrupt. Scary thing is, if I’d drawn a ton of pencil strokes, they’d all inherit the failure-to-flatten bug. So if you ever encounter the pencil-stroke-not-flattening issue, clear your Wacom preferences file.