If you are getting low OG, then you either have a problem with measurements, or you are getting low efficiency (or both).
Let's look at some of the more common measuring problems to start with:
- Are you measuring OG with a refractometer? If so, have you cross checked the results with a calibrated hydrometer?
- If you are measuring OG with a hydrometer:
- Have you checked the accuracy of the hydrometer be measuring the density of water at the calibration temperature?
- Do you take the sample at close to the calibration temperature (within 20F) and applied temperature correction if it is different than the calibration temperature?
- Do you always mix the wort thoroughly before taking the sample for checking? This is very important for a pre-boil sample, or after topping off with water.
- How do you judge volumes? Be aware that volumes at boiling point are about 4% higher than those at fermentation temperatures. Also be aware that the volume markings on ale pails are ofter very inaccurate.
If your measurements are good, then you need to check your efficiency. http://braukaiser.com/wiki/index.php?title=Understanding_Efficiency
gives some excellent advice in diagnosing the cause of efficiency problems.
From your OP, it looks as though you are fly sparging. This can work well with the right equipment, and if you take long enough to sparge; but can be a disaster if you use a braid or improperly designed manifold to collect the wort or if you sparge too quickly. I fly sparge with a cylindrical cooler and a false bottom. With a grain bill like yours, it would take me about 90 minutes to sparge.
Batch sparging may lose a few percentage points in efficiency vs a set-up optimized for fly sparging, but is much faster, and not so equipment and procedure related.
I found when I started doing a mash out before fly sparging (adding 5 - 6 qt boiling water, and stirring really well), my mash/lauter efficiency increased from 75% to 85%
The last thing is equipment losses. I typically get 85% mash/lauter efficiency, but only about 75% brewhouse efficiency. The reason is I lose wort to hop absorption, trub, dead space in equipment, wort trapped in the CFC etc.
Whatever, the first step is to determine where you are losing efficiency. Once you know that, you can take steps to address the problem, but if you don't know what the problem is, it is unlikely that you will be able to fix it.