It cant fire on just one hob its just that you would only have noticed it firing on the one at first, possible damp/sticking connection, worked on alot of these hobs in the past, sometimes they stop themselves sometimes it needs a new generator (the switch and generator are 1 unit) its not a diy job esp. due to the construction of these hobs so its up to you what to do next, can be a costly repair on these though so get proper quotations before committing.