It could also be you are (inadvertently) long-arcing and you don't realize it. To me, regardless of what root opening or land size you use, a fully penetrated root weld or the face side of a vee groove shouldn't fail, IF everything was done in accordence with the WPS and materials used are also correct. Check the parameters: electrode specification, travel progression, amps- min-max, etc. So , assuming everything is welded in accordance with the WPS then what else can we look at? How about technique? If we look at the root failure first, you need to examine very throroughly where the weld cracked. Was it right in the middle of the weld or was it more to the edge side where it ties into the base metal? IF it was right in the middle of the weld (assuming it looked alright before cutting and preping it for bending), then you would have to be long arcing and causing the the deposited metal (from the 6010) to become overly brittle due to improper arc length. IF it was on the edge or toe of the weld, then probably your rod angle was off and perhaps you where long arcing on that particular edge of metal. So in the horizontal weld and flat welds you need a 10-15 degree rod angle in the direction of travel and no more then 5-15 degrees  to perpendicular where (if anything) the rod should be slightly A-B-O-V-E the horizontal plane. Interesting that you have issues in the flat & horizontal positions since they are both similar in how they are welded. Transfer data on  how you do the horizontal weld to data on the flat weld and you'll see how much bead placement is identical.
With the face bends, you'll really have to do the same thing as far as determining where exactly the weld is cracking/failing. Since we are not physically there to see your test specimens then you'll have to be really throrough as to where the actual issue is. Does the crack (face bend) look like the root cracks??? OR are they somehow different? Usually 7018 laid down properly no matter what position...........it doesn't crack. So, are you cleaning each and every pass completely???? IF you are then (and you are sure) then you must be long-arcing this face or cap weld. I find this hard to believe because you'd notice alot of spatter and basically your bead would look like crap. Welding in the flat & horizontal positions allows you to run your heat on the fill/cap passes higher than other positions like for 3/32 "_ 7018, I'd run 92-105 amps. YOU can tell whether it is enough by what the rod looks like as it burns down as the last 2 inches of rod should just start to turn red. If the last 6 inches of rod is turning red then ofcourse you are too hot. These things are stuff your instructor may or may not have told you, but now you know so be observant and soon everything will come together. Good luck with that.