Question: “At many of our manufacturing sites we are reaching a point in our safety journey that they are struggling to understand. The issue is keeping people interested because they are forgetting the reasons we do everything we do in our safety systems.  The longer we go without an injury or very few minor injuries, the more push back we get on keeping the momentum going on our safety processes and introducing new processes.”

Response: By "Momentum" you essentially mean desirable frequency and quality of behaviors related to your safety processes right?  I'm assuming these are behaviors like getting permits, turning in near misses and hazards, and conducting JSAs as well as personal safety behaviors such as wearing proper PPE and following safety procedures.

Behaviors increase and are refined based on reinforcement.  However, the "lack of injuries" is not a reinforcer and does not increase or even maintain behaviors in your safety process.  So you're right that you tend to get complacency when you hit the plateau where there are few injuries.

When you have an injury, the injury itself create a context where safety behaviors tend to increase to avoid personal injuries and sad consequences such as seeing a friend hurt or hearing a supervisor gripe (negative reinforcement).  

Fortunately, reinforcement works!  Instead of relying on injury outcome data to reinforce behaviors in safety processes, you'll need to reinforce your safety behaviors more directly.  

1) Don’t assume your workforce knows what they need to do.  Define the specific behaviors you want to see.  Go past “following safety procedures” to include near miss reporting, harazard identification, peer-to-peer observations, proper permitting, giving a safety talk, etc.

2) Make sure your leading-indicator measurement system is strong.  Measure performance within your safety processes in the behaviors you defined above.

3) Then provide feedback abundantly via visibility boards, team meetings, and leadership "thank you's".  You may even consider non-contingent celebrations.