Undershooting OG in all-grain brewing

Great answer from brewchez. The other thing to consider (after the 2 he mentioned) is your water. If the pH is way off it can affect conversion. But crush is always the first place to look. One other thing to consider is the source of the recipes. Maybe they're written for higher efficiency than your system gets. You should always adjust any recipe to your own efficiency.

Two most prevalent issues for poor efficiency when batch sparging are 1) grain crush and 2) void volume in the tun.

1) Take a real close look at your crush. Crush it twice if necessary. If you are getting your grain pre crushed through a mail order, I'd invest in a mill and start doing it at home. My LHBS has a mill that is fixed to a certain gap. Through experience, if I think the grain is not crushed enough, I'll put it through twice and tell the guys its time to tune up the mill. Find some fellow brewers that do all grain and see if you can see their crush so you can compare it to yours.

2) If you put say 4 gallon of water in your mash tun and drain it, you should get close to 4 gallons out, say +/- a quart or so. If you get significantly less that's sugar you can't pull out of the tun no matter what you do. So how your manifold or valve (if you are using a braid) sits near the bottom of the tun can have a big impact. 1050 vs 1070 is a ~28% difference pretty big (so I'd suspect crush vs this issue). From 4 gallons or wort as an example that would be like leaving more than 4 quarts in the mash. This is the only time that mash tun geometry comes into play in a batch sparge set-up. You need to shoot for an 85-90% drain of the tun with just water.