Could my malt have been defective?

OK, I know what you are all thinking, but I am a regular all-grain homebrewer and have conducted a lot of mashes. Lat weekend I made a recipe (from this database, in-fact) that used about 4# of base malt, 4.5 # of Maris Otter (more or less base malt, I know), a # of crystal, a # of vienna and a half-pound of carapils. With the exception of the maris otter all was left over and posed no problems in previous mashes. The maris otter was purchased in bulk from my local HBS store just days before brew-day.
Mash temps were in range nicely (154 F). (yes, yes, about 1.5 quarts per #, pH good) but after an hour it still looked unconverted, and iodine test confirmed this. Bottom line is it took 2.5 hours and almost didn't run off. Ultimately I got 70% efficiency when I usually get 80.
No sour grapes, the stuff is fermenting away as I write, but I really want to understand what went haywire and I can't figure it out unless the malt's diastatic power was lacking or non-existent.
What are the chances of something like that? I drank too much beer waiting for conversion and want to know how to do that again

 

If you have some of the MO left, do a mini-mash on the stovetop and see if the numbers make sense.

 

Look at the modification of the Maris Otter. Take 10 - 20 kernels and check that the acrospire is at least 75% of the length of the kernel, preferably 100%.

Small portions of large maltings may not be 100% modified, which can lower the DP, among other things.

 

I know you probably already thought about this, but do a check on your thermometer. I had two that I used in the past that were both off big time... I missed conversions all the time as a result. Check the thermometer and check if often

 

The chances of getting a bag of bad malt are probably in line with buying a winning lottery ticket. How does a mash "look" unconverted? The most likely problem is a false negative on the iodine test. IMO those tests are next to worthless as most modern malts, especially UK malts, would probably convert if you relieved yourself on them. In addition to checking your thermometer's accuracy as was already suggested a poor crush is often the cause of lower conversion efficiency.