Two things:

1. The shield needs to go up generously before Levi casts the ult. A good 3s or more is preferable. This is a common issue.

2. There is a rare bug where the last orb dying does not stop the drain on the converter. It will keep draining for another 20-30s, leaving you at maybe 15-18 charge by the time Levi does his ult. I suspect this is due to the orbs dying during dives or some other bug. It's pretty rare, but I watch for it and call for a tank LB if necessary.