When printing a brim, does the filament not stick at all? Or does it stick on the initial pass, but get peeled off the bed by later passes? I’ve had issues with the v1 over-extruding, which forced previously adhered lines up off the bed. The problem seems to be worse at higher speeds (faster extrusion).
There are a few raft parameters for the raft that you can tweak in Luban. First, you have to customize your Printing Settings. Then you have to copy an existing profile by selecting it from the Profile drop down, then clicking the “plus” icon below it:
Now you’ll have a Profile named #Fast Print, and it will allow you to expand the Adhesion section, switch between the different types, and control a few parameters.
Unfortunately, pretty much the only parameter Luban allows you to change is how much larger the raft is than the print. To get more granular control, you’ll have to switch to Cura or another slicer.
But I’m hoping we can solve your adhesion problems by tuning the over/under extrustion. Using the same process to customize a print, if you open the first section titled Quality, one of the parameters you can adjust is Initial Layer Line Width. This is how much extra filament is extruded on the first layer. If you’re under extruding (nothing sticks to the bed), try increasing it and test print a brim again. If you’re over extruding (it sticks, then peels off as future filament is printed), try reducing it.
My v1 is manually calibrated to print well with Initial Layer Line Width at 100% on High Quality mode. But if I want to print with a Layer Height of 0.4mm, I have to reduce the Initial Layer Line Width to 75% (still experimenting). I don’t have a v2, but @xchrisd wrote a guide about getting started. In Section 7, there is a section named “EXTRUDER CALIBRATION”. It’s possible that this section will help you better than the Initial Layer Line Width adjustment.