One of our toilets began to flush slowly, and most of the water drains out, when it usually had filled the toilet about 1/5. I used a plunger, no luck. I purchased a closet auger, no luck. I have used suggestions from this site to clean out the syphon hole in the toilet, holes in the rims, adjust the linkage, still no luck. Water in tank appears to be okay as suggested on this web site. I am stuck what to do next.

One of our toilets began to flush slowly, and most of the water drains out,
You've checked everything but the vent. A clogged vent would suction the water out of the bowl. The solution would ne to snake put the lavatory vent. Put out enough cable to reach the base and 26 feet more. Good luck and thank you for rating my answer. Tom