If your ping test comes up with timeouts, wait a little longer and try again. I’ve found that some products seem to take awhile – up to a minute in some cases – to establish a WDS link, even after they appear to have fully booted.
Next, double check the settings on both APs. In particular, make sure you’ve entered the MAC address of the opposite end of the WDS link in each AP’s allowed MAC address list. Also make sure that the MAC addresses themselves have been correctly entered. You don’t have to worry about confusing O’s and zeros or I’s and ones, since MAC addresses use only the letters A through F. But I’ve been bitten more than once from reading B’s as 8′s (and vice versa)!
Then try power cycling both APs. Power down both, then power up the local (LAN-connected) one first and wait for it to come fully back up. Then power up the remote AP and wait for it to fully reboot. Then try the ping – again making sure you wait long enough before trying.
If neither of these steps work and you’re using APs from different manufacturers, you unfortunately may have found two products that won’t work together. In this case, there’s not much you can do besides trying another product, preferably another one of the same model for best chance of success.