Pre and Post OG's not hitting target

Hello, new to the forum.  I am doing 2 to 2.5 bbl batches and have been having issues hitting my pre and post boil OG targets.  I typically mash for 40 minutes and then recirculate for 20 min.  Mash temp depends on style but any where from 152, 154 or 156 F.  I measure my PH of mash as well and it is within the expected mash PH range.  I am trying to pin down the problem and wondering of your feedback on the following:

1. preground malt.  I rarely mill my own and am trying to pin down just one supplier.  As of now I bounce between BSG, Country Malt, and a couple of other smaller suppliers and have it preground by them all unless it will cause a huge delay.
2. Mash time
3. I sparge based on what Beersmith 2 tells me to, but opinions on other methods and feedback on channeling would be helpful.  How to prevent the channeling. 
4. My system is quoted as a 2.5 barrel system but the majority of my PB volumes are 84 to 86 gallons in a 100 gallon pot which makes for a rigorous boil.

If you have other questions or need further to provide feedback, please let me know.  Thanks!

The first place to start is to update your equipment and mash profiles to match what you are actually attaining.  If you have entered your measured values for a brew into the 'session' tab, BeerSmith will calculate your actual total efficiency and mash efficiency from these measurements.  Use these calculated figures to update your equipment profile.  If you have several brews, look for consistency in these numbers to reflect what you are most likely to attain.

When it comes to mash profiles, make one that meets your needs for each of the temperature rests you perform.  You can generally optimize your extract from the grains when batch sparging by shifting your water volumes between sparge water and infusion water to get approximately equal runnings from each.  If you are fly sparging, this is not important to make this change and the critical factor is the time taken to sparge. Longer sparges when fly sparging usually corresponds with a higher sugar extract and, therefore, yield. 

Since you commented about channeling, I will assume that you are fly sparging.  I don't do much fly sparging, so maybe someone else who does can jump in here.  The times that I have tried it, the critical factor was starting the draining of the wort slowly to gently set a good filter bed of grains hulls and particles.  Too fast and the wort will find a quicker route, opening up channels in the grain bed where there is the least resistance to the flow.

After that, when you make a change in the items you listed, you will be able to tell what impact it has made in your process.

Other feedback:  Country Malt, and BSG are distributors.  What is really critical is the maltster who created the malt.  For example: I can get Rahr 2-row from either source, but if they are from the same lot, they should and usually do behave the same.  There will be a little variability between lots, but this has been really minor in the past.  Moving from a Rahr 2-row to a Briess 2-row or pale malt would be a much greater change due to the change in maltster's processing and raw materials.

You say you are getting pre-ground malts.  How long are they sitting around and in what environment before you use them?  I grind my malts usually within 12 hours of using them. They are stored in a cool, dry environment until being weighed out and then kept in sealed containers following the grind to ensure they don't pick up moisture.

Mash time has some impact on extraction of sugars which is also tied greatly to the coarseness of your grind..  The more surface area which is opened up to the mash water (finer grind = more surface area) the faster the extraction of starches and conversion to sugars will occur.  Coarser grinds (larger particle sizes = less surface area) means a longer time for the water to saturate the particles and the slower the extraction of starches for the enzymes to work on.  Each process is a bit different based upon the brewer's grind quality, raw material quality, method of water introduction, and agitation to break apart small clumps of grain and wet them thoroughly.

Generally, in my process I can get away with a finer grind, but still mash for a full 60 minutes.  Extraction reaches a plateau near 45 to 50 minutes and the added 10 minutes helps to make sure I am not running my process on the edge where a slight change in raw materials can throw off my expected yield from the mash.

I hope some of this helps!

Consider changing to a batch sparge.

Read Denny Conn's page at http://dennybrew.com/ for a thorough guide to the process. It will eliminate channeling concerns.