One cause of your problem may be (or have been the last time) that you probably have nearly 8cf of peat moss compressed into a 3.8 cf bag, like the bag that I bought this spring. If this is the case then what happened to your most recent set of boxes is probably that the mix in them is composed of twice as much peat moss as you expected and is short on the other components, particularly the compost (where the nutrients are).

I found that the easiest thing to do, when I made my mix this spring, was to not rely on the measurements on any of the packages, but to use a bucket to physically measure the mix, one bucket peat moss (you will be amazed at how much it fluffs up when you take it out of the compressed bag and put it into the bucket) 1 bucket compost (I ended up getting extra compost, as the bucket method revealed that the bags, which were often labeled in pounds or other non cf units, frequently contained less compost than I thought (it's very hard to tell with all the different shaped/sized bags)) and 1 bucket vermiculite (which may or may not be compressed in its package, depending on the manufacturer). Anyway, building my mix this way I found I was a couple bags short of compost and a little short on vermiculite, so I went and got some more of each.

For the bed you describe you should need 21 and 1/3 cubic feet of mix. My own experience also says to make some extra, as it will settle when you water it, and settle more with time. I'd shoot for about 24 cf of mix. If your peat is compressed (as I suspect, you have nearly 8 cf (7.4 I think, you can make up the other half cf with one of the small bags, or just have a little left over after you mix, if it isn't compressed you will need a second bag and have 7.8 cf) two bags of vermiculite (7 cf total) and five bags of compost (I bet you will need more, despite what the bags may say, it's a good oppertunity to get several types and mix them). If you measure using the bucket method you should get a good, even, mix, but you may be surprised (probably unpleasantly) at the actual quantities of the different materials you got compared to what the packages say...

If you have "almost enough" I would recommend mixing in more compost to make up the difference rather than more of the other ingredients.