We only recommend fertilizer for healthy plants, and that should be applied in very dilute form.
Also, we find that Nepenthes recover from heat stress quickly. As soon as our Florida nights begin to cool down in fall, our more highland derived plants will show signs of recovery within a week. The leaves plump up and the new growth will be large and strong.
There are other factors that may cause a lack of pitchers, such as too low of a light level (did you provide extra shade to protect them from the harsh conditions?), and not enough humidity. N. khasiana is like what NepMaster stated: a real trooper. We can grow it here in south Florida outside with no shade cloth-even through the summer, and get large pitchers even with mid day humidity drops.
Look at what you did during the harshness of summer to protect the Neps, and maybe change the conditions back to the way they were before when they were happy. Also, figure you will need to protect them again with the arrival of summer.
Trent