Okay but you said "We covered propper throwing at a few practices" This should be done and reinforced at all practices for most younger teams, not just a few practices. Some will be more athletic than others. Reps, Reps, Reps and more reps. Once you understand this, your frustration levels and expectation levels will lessen. I have seen D1 players that have horrible mechanics yet they are playing college ball. At it's okay to stop practice, point out the flaw and reinforce proper technique. Ask them how it feels and remember that feeling when they do it properly. Praise good technique and celebrate it when they get it. I think I started getting my grey hair when my DD1 started her first 10U season.I never said I expected them to have it perfected after one practice, nor did I. (Even if that was not clear in my post) I just described what I saw and asked for some ideas to speed things along.
Had a former CEO tell me about the "Barf method". Tell/show something over and over until they barf...then tell them one more time!