I have never tried to get F upgrade / award on FRA-JFK so I can not comment on that route.
However on FRA-JNB (A380) it did not work out, even though I registered for the upgrade 2 months in advance and was told all the time (until one day before departure!) that chances are very good.
My theory is, that at the check-in on that day a group of HONs showed up together and went on a happy trip to South Africa taking away all the upgrade availability

.
So honestly, it seems to be very hard / impossible to get F award upgrades on A 380 unless you are a HON, SEN is not enough...
Good Luck in trying!