You simply ran out of WEP during your long sorties.
The Tempest has a maximum total WEP time of 8 minutes (max continous of 5mins, 'cooldown' 10mins nominally), the Spit XIV a maximum total WEP time of 10 minutes (max continous 5mins, cooldown 17 mins)
Once the total WEP time has been used up, it's gone. You got to start a new sortie (=fresh plane) if you want to have WEP again.