When a retaining wall bulges, leans, or drops a course, most people blame the block. In western Washington, the real culprit is almost always water. Months of rain saturate the soil behind the wall, and saturated soil pushes far harder than dry soil does. A wall that wasn't built to relieve that pressure eventually gives way to it.
A wall that lasts starts below ground. The trench gets excavated to firm soil, then filled with crushed rock compacted in layers. The first course of block sits on that base dead level, often partly buried. Skimp here and nothing above it will stay straight.
Behind the wall goes clean drain rock, not the native clay that came out of the hole, along with a perforated drain pipe at the base that carries water out to daylight. Filter fabric keeps the soil from clogging the rock. Taller walls also get geogrid reinforcement layered into the backfill to tie the wall to the hillside.
If you're getting quotes, ask each contractor what goes behind the wall and where the water exits. If the answer is vague, keep looking. A properly built wall costs a little more up front and a lot less than rebuilding one.