I'm in line with your comment more than the parent. This happened to me too. Turns out this expert was very knowledgeable, but spread very thin so he could never help out as much as was needed. Furthermore, there were just issues that one couldn't have predicted because it would require months of working in the code base to see the problems clearly manifest.

Now the guy is gone and we are too invested to be able to fix any fundamental issues without incurring pain, so it's not done.