I don't know all the reasons, but in my years of gardening I've had more trouble at times with spinach than with about any other thing.
I do think I've done the best when I plant in early August for a fall crop. One year I even made a mini greenhouse over the raised bed the spinach was in and I picked spinach all through the winter.
The only time I have great success with spinach is when the soil is about 70/30 dirt to horse manure. I add the manure in the fall...till it up real good and let it winter. Plant the spinach and other greens in mid spring and they do pretty well. Years I did not re apply the manure, were years they didn't do well.