“Sustain is perhaps the most misunderstood aspect of continuous improvement. Not only is the work to sustain just as hard as the work that created the gains in the first place, but the difficulties here tend to catch people off guard, leaving them confused and discouraged. ‘We were doing so well–what happened?’ is something we hear in a lot of companies.” 

The Toyota Engagement Equation by Tracey & Ernie Richardson (McGraw Hill, 2017) p. 175.