agreed. pc's in general are not that great. I've got 4x96w on my 40 and anything much more than like a foot away from the top doesnt grow nearly as well as it would up higher. And then there's the fact that mine a 96's and you've only got a single 36. I've never had the 36's but maybe they're less intense (they dont penetrate as much as the stronger ones) and might not even be good for that deep.
What sps have you tried and how much flow do you have, what are your levels, and where were the sps placed?