Can't jump using up & left key together

CKEN

Whenever I use the up & left key together I can't jump.

InternetPerson

Using a laptop? Update your keyboard drivers.

DanDK

It's a common issue tied to hardware compatibility. You can either rebind your jump / movement keys or get a newer keyboard that supports registering multiple keypresses (key rollover).

Thendriks

http://forums.maplestory2.nexon.net/discussion/comment/139651/#Comment_139651

@CKEN
Here is another thread regarding your problem with additional info.