A great retrospective is great (duh!), but it's only half the way. Retrospectives are a means to an end and that end is change, hopefully improvement.
If your teams are not committing to action items or aren't implementing them, don't worry. This book has got you covered. You will learn different ways to address problematic behaviours, how to engage everyone during retrospectives and help your team hone in on and follow-through on action items.