I recently bought a 2.5-12V 4-Channel HG7881 DC Stepper Motor Driver Controller Board for Arduino from eBay. All the 3 connectors are working fine that is they can make the motors rotate in either directions. But the motorD connection can only turn on one direction. what could be the issue here?
Check for solder bridges or other poor workmanship.
Double check YOUR wiring from start to finish.
If everything else checks out OK and you didn't accidentally fry it in some way then it was bad from the word go.
I have two on order and they should be here any day. Almost always worth double up on anything from China but for the most part have been pleasantly surprised by how well almost all of it has worked.