How wide and deep is the "greenhouse"? A single 2' T5 HO lamp with a good reflector is usually pretty good for an area roughly 30" x 12"; without a reflector, you are really only getting a small portion of the light output doing any good.
I actually measured the intensity of a Hydrofarm 2' T5 with-, and without its reflector, and saw this:
In fact the '0" from the reflector' point was actually 1" from the bulb, but you can see 1) the effect of the reflector, and 2) how rapidly the light intensity drops off with distance.