Having an accident Investigation scale dependent on the event From my perspective I think there are a several things to consider here.
Investigation process - why not get a member of the H&S committee involved in the investigation, investigations are best reviewed by a team of at least 2, so the line manager and the H&S committee member might be a good combination.
Outcomes - when the investigation is conducted is it looking at what was put in place previously and considering the effectiveness of those preventative measures? if you're having repeat occurrences, it may be that the measures have simply not been robust enough or even may not have tackled the causal factors.
Communication - it sounds as if your formula has been to integrate changes and results in toolbox talks, but repeating messages like this does have a negative effect, perhaps try an alternate mechanism - brief the team on the incident and ask them what would work better, get the person involved to talk about what they would do differently.
Trend analysis - sounds as if you have a trend, what is your company's approach to trend analysis and considering the wider implications? Consider all aspects of the work environment, and again, engage with team on what they need in order to stop the incident happening again.
Finally, in terms of engagement, toolbox talks absolutely have their place, but perhaps alternative mechanisms might be considered in addition, different people absorb information in different ways and at different rates, so try to have several ways of sharing the message.
Hope that's helpful