Some steppers only rotate one way on RAMPS 1.4
As towe said, it could be a dir pin ( can be checked by metering the dir connection), but in my experience this behavior has usually been a limit switch issue (as 0scar pointed out).  Try seeing if you have a limit switch setting somewhere that expects normally closed (assuming no limit switches are connected).

There is also another possibility that has to do with microstepping on some drivers, but i'm guessing you are going for a rudimentary no microstepping test right now (and have configured jumpers appropriately).