One thing I often see are vents that birds have moved into. It doesn't take much to prevent them!
Vent covers now come with a bird guard.
Most vent covers are flaps, either single or multiple. Birds are adept at getting into either one.
In a recent neighborhood of condos, only 8 years old, just from the parking lot it looked to me like birds were occupying nearly every vent!
You can see when they are present.
Sometimes bits of grass and debris will be sticking out.
Usually there are bird droppings on the siding or underneath. If you watch long enough, especially in the morning, you will probably see birds flying in and out.
Their presence is obvious.
And it can be dangerous! They move in and do not create cutsie little nests.
Their nest completely block the tubing so nothing can escape from inside.
The bath vent will draw air out one side and blow it back in the other. This can be demonstrated with toilet paper.
If it is a dryer vent that is clogged, well, the dryer can overheat and start a fire. Blocked dryer vents cause tens of thousands of fires every year.
BIRDS ARE OFTEN THE CULPRITS!
In trying to demonstrate to my client what she needed to so, I finally found a vent that has a bird guard. It was way up high, and glued over the old vent and flap with caulking.
But the problem is solved!
Such covers are NOT a good idea over dryer vents! And especially up high! They will attract and accumulate lint, effectively clogging the vent, causing a dangerous circumstance.
Don't mess with the dryer vent's ability to blow out air!
My recommendation: as a bird guard, those covers work great! I highly recommend them. They are made now as a complete unit, so your old vent cover can be replaced with the newer, guarded cover.