Could be a bad bottle of strips.

Many things effect those:

1) Water vapor from showers
2) Age
3) Exercise does burn ketones to which there is none to spill over into the urine

So it could be you are burning all the ketones by your workouts. We do have an exercise forum here both BFL and CKD/Body Opus you may want to drop in there and give TrainerDan a shout out and see if he has any ideas