I'd just like to thank @rj713, as your "another possibility" was the correct one for me. I recently acquired a Wallstreet in extremely good quality because the hard drive broke down early on and was never used after that. So I replaced the hard drive, which triggered this glitch with regard to the sleep switch.
After I held a small magnet next to the case for a few seconds, I was able to finally wake my computer from sleep. My mouth fell open from surprise. Didn't expect it to work, but it did. Thanks!
edit: in case the original link goes down: replacing the hard drive might generate a magnetic field that's slightly too strong for the Wallstreet's design. When you close the cover while it's on, it will not wake up from sleep mode, and if you close it after shutting down, it will not start up. You will hear an "electronic clicking" sound when you press the power button. Resetting the PMU won't help. Simply holding a regular magnet (I used my MagSafe 2 converter) against the case next to the delete button can 'reset' it.