I take a thermos of tea to rehearsal every night. But I've run into problems with making a second batch.

I use a tea infuser that fits nicely inside the thermos. I heat up water before I leave, and pour it into the thermos with the infuser, then it brews during the walk.

Halfway through, I open the thermos and pour in another cup of hot water. But this second cup doesn't brew very well, and I get very weak tea. I can't figure out what's happening. Do the leaves need to dry out before re steeping? Do they need air? Should I not be screwing the lid on while it re steeps?

Help!