Two year old condo we have downstairs unit on slab. Recently just flushing clear water will sometimes take two flushes. Water always goes down but sometime there is no strong push on first try. Periodically we get a sewer smell. The toilet sits tight (no rocking). I've tried bleach as well as using bleach in sink and tub. I've even poured down hot water from a bucket (that always works). The sink & tub do not appear to have a problem. All other sinks and bathrooms work fine.

Sometimes the wax ring gets flattened out and even though it won't leak because the horn in far enough down, when the wind blows, it will leak air out under the toilet bottom. As a test, caulk the floor joint. If the smell goes away, replace the wax ring. I do not permanently caulk the botton in case the toilet leaks since you might not see it and water will saturate the floor.

I agree, replace the wax ring, I'd skip the caulk and go right to about the only option, bad ring.