flanger not modulating.

I tried the flanger example sketch, and it plays the original signal, as well as a delayed signal as i would expect, but there is no modulation of the delay time with the built in sine oscillator. (so it is basically just working as a resonator)

I tried manually changing the offset index with it mapped to a control potentiometer which works, but all i can do is change the delay time by hand, there is still no modulation.

thanks