All four burners do not light consistently. Many times I need to use a match to get them to light. I can see a spark on each of the ignitors but today only one burner will light on each side; the other two burners just click but don't ignite. At other times, the opposite burner will light. There doens't seem to be any consistancy other than all will light if a match is used. I was wondering what parts I need to replace?