A few thoughts:
1) The "creeping maximum reduction" is probably caused by the pedal's autocalibration – if you hit it very hard, it will view this as a maximum (100%), and thereafter it will be hard to reach high values without hitting it equally hard. This implies that the pedal doesn't normally reach its maximum under gentler use, so some adjustment of the pedal end stop may be helpful.
2) When the pedal is only operative over part of its range, this suggests that the pedal's pot has too high a resistance – it should be an expression-type pedal of about 25K, whereas a guitar-type volume pedal may be 250K. Note that some pedals have a range adjustment knob, which can be useful.
