I spent part of the evening last night reading about "retarded souls" in The Three Ages of the Interior Life by Reginald Garrigou-Lagrange, OP. Very interesting!
One of the indications of a dying religious order or community is when the number of fervent souls is outnumbered by the number of "retarded souls." Another name for a "retarded soul" would be a tepid soul. Basically, a person who after their initial conversion starts slipping backwards, allowing their ego to once again dominate vs. God eventually just kind of drifts off, and doesn't grow closer to God (obviously).
I found it fascinating not only for the self-application but because it is something in a world where the "world" is always spreading its version of the gospel so many make no progress in the spiritual life these days.