It's worth noting that the rheostatic can be switched on but the conditions for it to become active arn't met. It only applies if the camshaft has been notched up to at least full series or above. Some drivers I've noticed shut off power (having notched above full series meaning dynamic would become active when brakes applied), but then reapply power to a low notch and shut off again, meaning the camshaft notches back to the starting position/below full series. You then won't get rheostatic braking. Some drivers do this so they can set off from stations quicker as they don't have to wait for the camshaft to notch back down to the start position (so once the driver applies power from a stand, you can tell what position the camshaft was in by the number of contactor clicks and clunks before it actually sets off) - if there is just one clunk and away it goes, it wouldn't have been in a high enough notch for rheostatic to be active.
Also the rheostatic only kicks in at around 60mph and below, so if doing say 70mph it'll be disc braking only to around 60, and the dynamic will then takeover. Finally if the unit detects wheelslip under rheo braking, it'll cut it out and resort to disc braking only while the WSP (Wheel slip protection) does its thing.
Hope this helps!