I was watching an Olympic Recurve archer shoot group after group indoors whose height was excellent but the groups were 5-6X wider than they were tall. I finally broached the subject with him. In general, it is not recommended that coaches offer unsolicited advice (it is strongly recommended to not) but I had met the young man prior to this and I knew he had had some coaching in the past.
He apparently had had this problem for a very long time, which is sad because I told him his problem was simple: he need relax both hands (primarily his bowhand). You see, if your hands are tense it tends to cause the bow to jump ever so slightly left and right upon release. This causes a left-right dispersion. A very tiny movement of the bow (twisting it around the pivot point) moves the bowstring even more. A tiny (1mm/1/16˝) movement of the bowstring left or right is going to make several inches difference at 20 yd/18 m.
So, you coaches working with beginners, once you have established a reasonable full draw position for a student-archer, you need to next focus on them having relaxed hands. (Yes, even the string/release hand. The muscles crooking the fingers are in the upper forearm; the hand needs to be relaxed otherwise.) This is one more reason I recommend low draw weights for beginners. (I start adult beginners with a 10# recurve bow.) The less the strain on the archer, the easier it is to learn to relax. Also, to get off the bowstring when the draw weight is very low requires even more relaxation, so such bows give good feedback. Once the proper behaviors are learned, then the draw weight can be cranked up a little at a time while the archer focuses on retaining his/her good form and execution.
Let me know if you encounter puzzling situations you would like some help with.