Sold properties aren't found on MLS?

Hi Everyone,

I'm looking for comps to get an idea of what one of my appraisals may come in at and I found 5 good ones. Here's the thing, I used Zillow's recently sold feature to look for them because they have a really easy interface/map system, but when I later plugged those properties into my MLS they didn't show up as ever being sold or even listed. To make it even stranger, I found the same properties with the same sold information all over the other sites such as realtor.com, trulia etc etc. Basically, they are everywhere except the MLS. I'm pretty certain these properties did in fact sell and for some reason the records on my local MLS are incomplete or something. Anyone know what could be causing this discrepancy?