I have a front loading washer and after each cycle is finished, I can spin the tub and hear water sloshing.
I wash urine absorbing pads and I can never rid the washer of the smell. I can do twenty empty loads with various odor reducing chemicals and nothing helps. I've tried everything short of tossing it out the door? Do all front loaders retain water under the tub?
I don't have a front loader so I can't help you with that, but you may have luck soaking your pads in a bucket before laundering them. Start with 1/2 cup baking soda dissolved in the soaking water and see how that goes.
All washers retain a little water in the "boot". My advice would be to run a rinse cycle after your daqy's wash with a cup of vinegar added. Or you could run the pads through a rinse cycle before you actually run the wash and they would not have the urine still in them.
If you are using liquid detergent it leaves an odor. Follow the advice of the others and switch to powder. I use Tide HE powder and since I switched I have no odor.
I have a male cat who is occasionally incontenent so have lots of practice with urine. I use 1/3 cup borax (Borateem) in my front loader and never have a problem with odor in the bedding or the washer. I dont know what I would do without it. Good luck.
I have heard this is one of the complaints about front loaders and the musty moldy smells that retain as well. Never shut your door once you are done with laundry. That advice was from the laundromat owner. Air circulates. And with the smelly clothes that get brought into a professional place to be done, rather than their own machines is often.