As many moodlers say on several forum threads (e.g. https://moodle.org/mod/forum/discuss.php?d=146812
and https://moodle.org/mod/forum/discuss.php?d=146812#p647044), under unknown circumstances (so far) the grade from a quiz for given (not all) students keep not reflected on gradebook. Sometimes appear 0 and the majority you obtain no value ("-") displayed.
Wordaround so far:
1) Go to ITEM CATEGORY > Edit > Hidde > save changes
2) Go to ITEM CATEGORY > Edit > Show > save changes
3) Refresh (F5) main gradebook view and, magically, the well grade from quiz will appear!