I was going to say the same as moms below - leaving door open after every use. I used to always close them, and then had the same problem.
I have to clean mine out with bleach (run a load with just bleach) from time to time - that has helped us. Haven't tried the vinegar but sounds like a good alternative.
We also use less detergent than they recommend. That had a big effect for us. I had suds building up in the washer, and it was causing problems draining, etc. I was also told to do that with children with skin sensitivities. Mine have eczema and allergies so that's a must for us. The less detergent the better.
Are you drying your clothes fully? Where we don't use scented detergent, I find if our clothes get hung up or put away without being fully dry, they get that smell.
Great suggestion on cleaning the draining trap!
Good luck :)