Try using a different instruction - they probably think they are ear to ear, but happen to be fully engaged (that's front rows for you).
On Bind, shoulders apart please - wait for Set before you come together. No pushing before the ball is in.
First scrum, they will just keel into each other as they are used to at that level. Stop, remind them of the briefing, ask if that's clear and reset. Next scrum - thank them for complying (thanks guys, I knew you could do it - same again please). Works until they are completely kn*ckered. And most FR will comply; if they don't, ping them. They all know what they *should* be doing.
I agree that as the game proceeds at lower levels, tiredness will kick in - then it's contextual. Your choice to keep the game going as long as it is safe or ping; if the ball is coming out quickly, probably better for the match you are reffing to keep it going. Watch out for replacement FR, who may need similar treatment - stop on first one and remind them, don't just ping - I am talking lower levels here, 9 and below in UK.