why won't my armitage shanks dual flush work properly?

Hi - I have to push the buttons, both of them, several times, to get the flush to work. It's got so bad that I have pain in my thumb joint from the continued pressing. But it's not consistent. Sometimes I only have to press a few times, other times it's several times. The only way I can describe it as someone continually putting their fingers down their throat to throw up, you get retching the first few times, then *wooosh*. Any clues anyone?