Why is my indicator empty at the start?

An average of 200 bars has nothing to say for the first 199. Each function's reference entry gives its warm-up, and rules do not run until the script's warm-up has passed. The compiler works the warm-up out for you. Set warmup: in the header to override it.

A higher bar size also shows nothing new until its candle closes, by design. A 5-minute rule never sees a 4-hour bar early.