Toto CST703 Toilet Not Clearing

Our 3, 15 year old Toto 1.6 gal toilets have been having problems and need constant plunging and multi flushing. The plumber says it is the toilets themselves and not the plumbing. Any idea how long the suction on these is supposed to last? What are our options? Do we need to replace the toilets or a part inside? thanks

Has the performance changed? Did it do it all at once, or gradually?

Are you using a generic flapper valve or an original? Does it get better if you hold the handle down to use more water? What about the fill valve? Has that been changed? If you take a bucket of water and slowly pour it into the bowl, does the bowl level end up higher?

Do your showers and baths drain as fast as they did? Any strange gurgles or other noises from any drain?

If you have really hard water, the holes under the rim can become somewhat plugged. Check with a mirror. A wire from a coat hanger can often clean them out if plugged, but it could be any one of many other possibilities not related to the toilets themselves.

The CST703 was a builder grade toilet that TOTO sold.
MaP was 550 grams when new.

It had some tight bends on the lower trapway, so over time, you may have collected some stuff there.

You may be able to auger out the trapway.
Or if you have lime in the water, you may need to clear some rinse holes.
Either way, it was a $99 toilet to start with.