You said 5 m/hash. For 1.5 m/hash that's to be expected.
This is one of my two rigs. The second has 3600 kh...
Well two rigs totalling 5 m/hash isn't the same as 1 rig that mines at 5 m/hash.
I would expect the 3.6 m/hash to get a block on average every 5 hours, but that's just an estimate, and you can get lucky/unlucky.