My keypads will accept the first couple of keypresses and then appear not to work.
The most common cause of this issue is that you have two keypads addressed as the same address. You cannot address, for instance, both keypads as number '16'. They will cancel each other out and the keys won't function properly.
Another cause of this is that there is a loose wire in either the keypad itself or at the panel end. Another, lesser, reason this occurs is that the keypad may be older and the key contacts are worn out.