Did you just directly screw the boards to one another or to a supporting post within?
The first time I made some I had the same issue as I screwed the boards together. After that I constructed the walls with supporting posts internally at each end that has held pretty well. I don't know if I'm describing it well enough though!
Yeah I screwed the boards directly into each other... Yeah I see what you mean, that may have helped.. but its not the wood that failed.. it was the screws broke off from the force of the dirt in the beds I think.
Yeah screws can be iffy. I had some recently that just corroded into nothing within a year and I was a tad disappointed!