It's the plug and/or the water you are fishing it in. Mono sinks too, jusy not as fast as flouro. Not sure about the flouro sink thing, but maybe a leader stuck under water could have something to do with it.
A zara should never go under. Eye on the bottom and very buoyant. At least the small ones. Great lure. It should stay up even if 400lb test, 49 strand wire was attached to it.
Congrats on the keepah USS Blue.