It's impossible to say without further details. It might be some random glitch, although this would be the first time I've heard about it. The respective participants might have (mistakenly) quit the respective script(s) by pressing CTRL+Q.
Or you might have some error in your scripts, perhaps involving inappropriate /stop or /skip conditions, or inappropriate condition assignment (see below).
> it appears from the web log that they were never administered these scripts in the first place
Depending on how those scripts are set up / how you assign conditions (via the /subjects and /groupassignment attributes), there may be certain combinations where those conditions simply *do not apply* and hence the script isn't administered. As a simple example, suppose you have two <expt> elements set up like this:
/subjects = (1 of 2)
/groupassignment = subjectnumber
/subjects = (2 of 2)
/groupassignment = groupnumber
The first <expt> assigns based on *subjectnumber*, the second one uses *groupnumber*. The consequence is this: If an *odd* numerical subject id is provided, one should receive the blocks defined in the first <expt>. If the random groupnumber is *even*, the second <expt> might be run in addition. Now suppose the subject number is *even*. Consequently, the first <expt> doesn't apply then. Further suppose the groupnumber is *odd*, i.e., the second <expt> doesn't apply either. As a matter of
fact, in this case *no condition* applies, and the script would not be run at all.