Walk any neighborhood with hillside lots and you can spot the failing retaining walls from the street: a bulge halfway along the face, a course that has slid forward, caps that no longer line up. It is tempting to assume the wall was undersized for the soil it holds. Usually it was undersized for the water.
Dry soil pushes on a wall with a force you can calculate and design for. Saturated soil pushes far harder, because you are now holding back the weight of the soil plus the hydrostatic pressure of the water sitting in it. A wall that is comfortable holding a dry slope can be badly overloaded after a week of spring melt if that water has nowhere to go.
The fix is not a bigger block. It is drainage, and it is three things working together. First, clean angular gravel backfilled directly behind the wall so water falls straight down through it instead of soaking into the retained soil. Second, filter fabric separating that gravel from the native soil, so fines do not migrate in and silt the drainage rock into mud over a few seasons. Third, a perforated drain line at the base of the gravel that daylights somewhere real, downhill, at an outlet you can actually see.
That last part gets skipped constantly. A drain pipe that ends buried in the backfill is not a drain, it is a pipe. If nobody can point to where the water comes out, the wall does not have drainage.
Two more details separate a wall that lasts from one that does not. The base course should be buried, roughly an inch of embedment for every eight inches of exposed wall height, sitting on a compacted leveling pad rather than on native dirt. And on taller walls, or walls with a driveway, patio, or slope loading them from above, geogrid tied back into the retained soil is what turns a stack of block into a reinforced mass that behaves like one unit.
All of this is invisible once the caps are on. That is precisely why it is worth asking about before the first block is set.