Dog pee in each corner making permanent odors that will keep your house unrented.

It really works well for smoke, pet odors, food odors, etc.

The bylaws have a clause that no resident should allow noxious odors to come from their unit.The HOA president has inspected the stairwell when my tenants have complained and she also does not smell anything.My tenants keep contacting me about this and I am wondering what else, if anything, should I do?

When this happened in one of my buildings, I went into the complaining neighbor's unit and smelled the odor myself. 

No harm no foul.

You can use odor blocking primer in many cases, but in extreme cases you may need to rip out sub floor. 

Could be graffiti, gang signs, boarded up houses, loose dogs, undesirable neighbors (for example if people are out on their porches and look less than appealing or are outside screaming and fighting), lots of trash all over, a bunch of ancient vehicles, a nearby business or something that omits a bad odor, etc.

25% for flood when creek rises, fire, non paying tenant, roof repairs, tree falls on duplex and insurance deductible is $2500, tenant puts toy down toilet and clogs main line, dog which you did not allow bites someone, disposal stops working, property manager 10%, property taxes increase because they see you did rehab or you changed the title, noise fine as tenant decides to become a drummer, trash bill, ants or vermin and you have to hire a exterminator, mold claim, planning department inspection fail, city fee as business, legal fees, CPA fees, insurance goes up, Imperial Beach business license and landlord permit/ annual fire safety inspection/gas co inspection $1500+, heaven forbit the Coastal Commission says you dumped foul water, to name a few

If you don’t like it, no harm no foul.

You may get audited and the IRS may call a foul and hit you with a claim for the capital gains.