Caffeine pills don't dissolve in hot tea
If it really has to be pills, then I guess you need to mix in something that dissolves more easily than the caffeine powder. Powdered sugar seems like a good candidate. I found a couple references to people using it as a binding material for other pills, so presumably it'll work.

Depending on how you're making your tea, you could also add the pill while you're boiling the water, to give it more time to dissolve, and to make sure it gets the full heat.