To my knowledge, Cryptic has specifically said that missions that qualify need to have at least 5 reviews and have an average playtime of 20 minutes or more.
If missions that meet those qualifications aren't showing up as qualifying, it can mean one of two things. Either there are more qualifications that they haven't mentioned, or there is a bug.
The Foundry Roundtable live Wednesdays at 7:30PM EST/4:30PM PST on twitch.tv/thefoundryroundtable "I would rather take a chance on getting a change made to the game through positive engagement than know for sure that I got a change made by complaining" -drogyn1701, May 14, 2014.
The missions who weren't played at least 5 times don't give any credit for playing them. Check "review content", you'll see no mission is eligible there.
Hmm I can see it from both points of view.
On Cryptic's side:
Before people play it the game has no idea how long it takes and whether it would count. At the same time if it lets it through everything before the 5 reviews then it could easily be exploited. People can play it over and over without reviewing so it never leaves the review period.
For the Author's side:
Pretty simple, like you said how do we get the first 5 reviews?
Is there a safe middle ground? Or will we have to reply on people who go out of their way to play missions that do not reward them.