I just got my (self-assembly) Metal PrintrBot 2 weeks ago and I had a lot of problems getting the prints to reliably adhere to the build plate.
There were quite a lot of things I tried to get reliable adhesion, and now that I believe I have reasonable success, here is the list of what had to do.
The list is ordered by greatest significance - with the tricks that had the greatest success at the top.
1. Correct Leveling.
The PrintrBot auto-levelling firmware didn't quite work, and after a lot of testing I discovered that my build plate was slightly warped. By warped, I mean that when three corners of the plate were exactly the same distance from the extruder, the last corner was too far down by 0.2mm.
That 0.2mm meant that any plastic extruded anywhere near that corner would only be loosely pressed down, and would certainly lift-off within a few layers.
Solution: Add spring washers under the plate screws, and manually level the plate by testing at ALL corners.
Remove the build plate screws, add a flat washer on the plate mount screw holes with a spring washer above it.
Put the plate back on, and screw it lightly down so that the washers are just compressing.
Set the height of the extruder so that a flat metal test object JUST fits between it and the plate.
Move the extruder around and test the height at ALL corners of the plate - adjust the plate fixing screws up or down to make the plate completely level.
At this point, whenever the auto-leveling program runs, the bed should be so close to correct that there is very little error for the software to trim-out.
This method worked very well for me, at least.
2. Triple-loop skirt
In the slic3r software I set the skirt to 3 loops, at 12 mm away from the object
The 3 loops of plastic allows the extruder to fully prime with plastic that flows well.
3. Very slow first layer printing
In the slic3r software I set the speed of the first layer to 20%.
As the skirt is printed on the first layer, I watch it closely, and carefully remove any lumps on the plate with tweezers.
By the time the 3rd loop is printed, the extruded plastic is pressed smooth and flat against the plate, without and stray lumps to foul the object about to be printed.
3. Wide brim - for tall objects with a narrow base
In the slic3r software I set the brim to 10mm wide.
No Brim or small brims on tall objects don't tend to hold very well.