The requests seem legit, so you can't blame them.For example, my tenant told me that the toilet is clogging a lot, seeming like there is not enough flush power.

Turn on the water in the entire house, tub all faucets, and flush the toilets.

Plumbing shouldn't keep backing up for no reason....Are there small kids in the house flushing toys? 

If the plumber discovers the line is full of wet wipes or someone tried flushing a diaper, that's clearly abuse and the Tenant should pay for it.

And, do yourself a favor and don't list your property if it's not maintenance-free; do the work first so that no major issues arise - including replacing the toilet flush valve, air filters, light bulbs (if old), and ensuring there are no trip hazards on the property; have an entry process/system in place for guests. 

Excellent guidance, another hint to look that I have seen it few times is the addition with a step down from the house main floor or the detached addition flush with the ground, it need to be about 4 to 6 inches above ground to be a living space.

Every living unit must have at least one (1) bathroom which includes a flushing toilet, lavatory/sink and a bathtub/shower.

Now, however, I am becoming more attracted to the notion of trying to flip properties in my early years as much as possible, so I can generate a good starting cash base and a cash-flush business, then once that is established build a portfolio of rental properties to generate long-term stable cash flow. 

If you are just flush with cash then I would look in to tearing it down and building something bigger.

Start by inspecting all the toilets to see if one is constantly running/flushing and you'll probably find your culprit.In the future, stress this to your tenants.