I've only used the batch sparging method since I switched to all grain, and it did take me a while to dial in the process to get consistent efficiency every time (~75%).
I know that a lot of this has to do with your particular equipment, but it looks like you've got a similar setup and I'll tell you the things I did to try and help.
I'm not here to discount any of my fellow home brewers suggestions or tips, but deadspace and grain absorption are something we all account for in our particular systems and try to adjust for it when figuring our mash in and sparge volumes. Obviously, limiting the amount of deadspace is something that should be done, but is not something that can always be accomplished.
Here's what I have done:
1) I have my own cereal crusher and did have to adjust the gap down a little bit to get a slightly finer crush. I was using the "factory" setting when I first got it (I think it was at .042) and have now set it at .039".
2) I raised my water to grain ratio from 1.25 to 1.35 qt/lb. This made the mash thinner (obviously), but I think it mostly helps when I dough in. With the finer crush and the extra water, I find that the grains absorb the water more easily and allow more sugar conversion to happen.
3) After dough in, I stir the heck out of it. Now I'm not talking about madly beating and creating a frothy mess, I just take an extra few minutes to slowly stir and mix the grains into the water. I also do this after I add my sparge water in preparation for the second runnings.
4) I don't vorlauf. Now before anyone goes off on me, let me explain…I've found that, in my case, the methods I've tried to use in the past have not worked when I try to re-add the initial vorlauf runnings back onto the grain bed, too much grain bed disturbance for me. When I started to just empty the tun by filtering the wort thru a sanitized strainer/funnel, I found that I was getting a smoother draining process and after the first few quarts I was getting clear wort anyway.
5) Lastly, the only way you are going to be able to narrow down where your problem is specifically, is to take gravity readings thru-out the draining process. I do this by taking a refractometer reading every gallon while draining my tun. I record this in my brew log and compare it to my batch prep log. I do this thru-out the first and second runnings and am able to tell if I'm going to hit my target gravity and where my process has failed.
To get batch prep log expected gravity readings, you can use a couple of websites to determine what it should be. By entering your grain bill, batch size, and expected efficiency, you can calculate what your first and second gravity readings should be, as well as your target gravity for boil.
Here area couple of websites I've used in the past:http://tastybrew.com/calculators/sparge.htmlhttp://www.thebrewsite.com/batch-sparge/
I know there are more sites, but those are just a couple of the ones I use. Also, if you have any type of brewing software and have your equipment specs entered in, it should be able to give you those readings as well.
By incorporating these changes to my mashing process, I've been able to maintain a consistent efficiency and can identify when and if I'm going to have any problems with getting my target efficiency with a batch.
Hope this helps…