Old low-flow toilet won't flush without extra water
Bill Burnett,Kevin Burnett
Hey, they mention my website as part of the answer, cool!
SFGate, San Francisco Chronicle
After reading the article, I'm thinking 1993 is much of the problem. Toilets sold in 1993 were not very good.
There have been improvements since that time. However, even a 1993 should work most of the time. It may be that over the years, the lower trapway has been the happy collector of all things lost. If there are objects collecting in the lower dwelling places of the bowl, it can slow down the siphon. If it can be cleared out, you might be able to restore the feeble 1993 flush action. Sometimes, things are so hidden and wedged, that it's not possible to clear it though. If that is the case, or if you are just tired of feeble flushing, then buying a high performance flusher may do the job.