First of all...are you washing any loads in warm or hot water?....my SIL only washes in cold, and theres always a film of sludge (lint, grease, suds, detergent) built up between the inner and outter tub at the top, about 6 inches around, and I have to disassemble the base from the machine, take off the tub cover and scrub with a brush and hot water to get rid of it, also the dual action agitator is taken out and apart to clean out, TILEX DEEP ROOT spray works best on all of this, also remove the balance ring and clean under that too....
after that is all clean, rinsed and replaced, I fill the machine with HOT water and a full gallon of bleach(you may have to hold the reset button of the water level to fill as much as possible)...let agitate and soak for about 2 hours, then let it finish the cycle....while soaking, move and tilt the tub assembly to get water and bleach under the tub where the air is trapped, also drain the machine for a few seconds to get the bleached water into the pump and hoses, then refill
this should take care of this for a while.....keep the lid open when not in use to dry out
Super Clean Purple is also a great degreaser for this task, you can find it at Walmart, or any Auto Parts store!
Hope this helps you