How do I stop my 10&12 diving into the ruck

My10&12 have a tendency to dive into the Ruck leaving a huge hole that my oppose score through. I have tried lowering my DefSpeed on the players and tried various Def Spacing but nothing seems to help. Can someone tell me how to keep the 10&12 stay in position? Many Thanks.
Is your tackle and safe tackle maxed out for your level on both players? Have you looked at ruck aggression too maybe?

Personally I find the 10 running into the back of the lineout to be quite useful in terms of stopping the ball being shipped out from the back of the line throw.