best way to find out is to hook the ballast off the other side up to the bulb which is not working.
this will show if its the ballast or the bulb
has the same problem on my kit after 1 week, worked out the ballast was faulty as the other one lit up the bulb straight away.
got intouch with the guy on ebay, told him what id done to determine it was the ballast and he sent out a new one
thats presuming you got them from ebay
but yea, do what i said above and you will find the problem.