You must be doing something wrong...
I am mining with only one USB Eruptor and my current rate is:
Last hour rewards 29.91345551 ZET
That gives me currently about 720ZET per day with one USB Eruptor.
This rate is pretty constant too.
What pool are you mining?
I was mining on dsync but there was some kind of bug a couple days ago. That got me 44 in 4 hours. It appeared as zero then but after the bug was fixed the balance showed up and I went at it again. I don't know what the problem was after that. When my balance was 13 ZET after four hours I decided to let dsync sort out whatever was going on over there.
Doing it solo means maximum variance, and I seem to be getting my maximum variance good and hard. :-)