first off, i'm not an electrician but... i know enough about this subject.
your ballast may be bad. that's the obvious one..
the not so obvious is that there are a hundred different types of bulbs. i don't remember how they are rated at the moment. they aren't volts i don't think, and they have a cute name for whatever measurement they use.
long story short, you need to make sure your ballast matches with that type of bulb.
if the bulb is too big of whatever measurement, then it won't power up...
if its too small, you'll burn the bulbs up because you have too much power going to them. it usually has a tolerance of a size or two difference, but not much beyond that..
if you have a 35 ballast, putting a 50 in it will probably not work right.
if i HAD one of these things handy i'd be able to help more..
the way you can tell is by looking at the ballast and seeing what it's rated for. it usually has it ON the ballast, but you may have to take some stuff apart to get to it.
the bulbs should say on the box itself what they are rated for
i really wish i remembered what to tell you to look for, but...
once you look in there it will be more than obvious most likely.
if that's what's wrong you can either change out the ballasts (not a bad idea anyway) to match, or you can send the bulbs back.... either one will cost you money.
good luck