it's odd that that would happen. up and down are vastly different. let's try to isolate the problem.

check to see that the servo is moving the same amount both ways. flip the wing around and just take a look. if the servo is moving more one way than the other, then you know that it's a problem with the radio.

if the servo travel is the same both ways:

remove the linkages to the servo and check that you can physically move the ailerons to the 1/2" throw up and down. if you can't, then the ailerons are getting caught somewhere.

if you can move them to the desired throw easily, then lastly:

check with a throw meter and actually measure the throw. it might have all been just an optical illusion.

and yes, you can fly like that no problem! but still am interested to figure out why you have this problem!