How do you fix a leaky onboarding flow without killing your team's growth momentum?

When you realize that the vast majority of your new accounts are going dormant almost immediately, panic usually sets in. The knee-jerk reaction is to completely freeze feature development and overhaul everything, which usually just stalls the whole company. We had to figure out how to patch the onboarding leaks while still keeping things moving forward. For founders who managed to transition from top-of-funnel acquisition to real retention, what was the first actual change you made that worked?